This article is a travel topic
A hospitality exchange or home stay network is an organization that connects travelers with local residents in the cities they're visiting. If travelers can connect with the right people at the right time, they can get room and sometimes board in the place they're visiting for free or at a deep discount. Network size goes from a few thousands to a hundred thousands, and most networks are growing steadily.
Home stays have advantages and disadvantages. The most obvious advantage is that accommodation costs are much lower (at most networks for free) than at hotels or even youth hostels. More important, though, is the opportunity to make a personal connection with someone from a different culture and social classes, you see the destination you're visiting from a local perspective. You have your own adventure and as a side effect, the goal from some networks, this can build and strengthen intercultural understanding and reduce prejudices and intolerance.
There are disadvantages, though. Home stays require some additional planning before travel, and courtesy requires sticking at least reasonably close to your schedule. There are usually strict limits on the length of stay and what you can do in the home. And the opportunity to make a personal connection has its flipside: awkwardness between host and guest can make a visit to an otherwise pleasant city unbearable.
There are a number of different networks that connect hosts and guests, with different requirements for participation, restrictions for guests and member number and geographical spreading. Hospitality Club and CouchSurfing are the biggest online organizations with over 300 thousand members. The number of active members within the different networks is unknown due to a lack of standard definition of what exactly constitutes an active member.
For many, joining just requires filling out a Web form; some offer and others require further verification. Usually a Web listing or printed book of available hosts is provided, sometimes with eBay like reviews by travellers (or vice versa). All listed networks operate worldwide.
Here are how the various projects index by size:
Members * Organisation ------- ------------ 3,360,205 * #CouchSurfing Project 437,042 * #Hospitality Club 56,961 * #GlobalFreeLoaders 13,000 * #Servas 7,236 * #BeWelcome 4,000 * #Evergreen Bed and Breakfast Club 4,000 * #Affordable Travel Club 1,350 * #Pasporta Servo 700 * #Hospitality Exchange ? * #Casa Casa ? * #Belodged.com ? * #Stay4free
The CouchSurfing Project  is the largest, hospitality exchange organization, founded in January 2004. While it started off as a non-profit organization, in 2011 it became a for-profit corporation. As of July 2010, there are over 2,000,000 members in 70,000 cities of 236 countries. Membership is free. Profiles are very comprehensive and extended search is possible (Allowing to search for specific characteristics such as "wheelchair accesible"). One important feature of this site are the groups. The groups allow people create "posts", make questions and interact frequently, creating a stong sense of community. The groups are set by interest (cyclist, vegetarians, alternative consumers,etc) or by location. There are several security measures. After using the service, you can comment ("leave a reference") about your host or guest. You can create links to people you've met through a friend list. It is optional to become "verified member" (one-time fee, amount depends on your home country) providing another layer of security and keep the project running. Another safety system like verifying is called vouching, in this system you can vouch for others you know and feel are trustworthy if you yourself have been vouched for by 3 other members. This system was started by allowing the founders and core admins to vouche for others from the start and it is spreading pretty well. It is possible to see if people are traveling themselves, and the percentage of messages responded to. Despite a system failure in June 2006, the majority of the data was recovered, and the system restored to functionality. There is also a wiki on the site with helpful notes about couchsurfing, safety, countries, cities and other couchsurfing related topics.
Unlike the next largest hospitality exchange, hospitalityclub, couchsurfing is much more focused on "social networking", and members organize many events, viewable in a database on the website. Alongside with the surfing/hosting expereince, events are good oportunities to interact with people living or traveling in the places you visit.
CS mission is to facilitate inspiring expereinces. CS vison states that the creation of meaningful conections across cultures leads to appreciation of diversity and ultimately to a feeling of community and more understanding among people.
The Hospitality Club  is the big hospitality exchange website which was founded in July 2000. As of August 2008 there are over 437,000 members in 207 countries. Membership is free, and each member is verified by a volunteer team.
To register a member must provide their full name and address, which is verified by volunteers. Potential guests can either navigate the database of hosts geographically or use the advanced search feature. An internal message-sending mechanism is then used, allowing to keep email addresses confidential and to block spam thanks to checking by volunteers. Individuals are free to arrange their own conditions for the exchange within the rules of HC that include the hospitality exchange must be free, but allows for ancillary costs such as food and phone calls to be privately agreed between members.
After using the service you can leave a comment for a member which will be visible to all users. The site also includes very active forums, groups, and wiki style travel guides which members can update with local information. In addition, Hospitality Club has the most active group activities, with regular regional meetings and huge camps with sometimes over 400 members attending. The club is based on the work of volunteers around the world who believe that by bringing people together they can increase intercultural understanding and peace.
GlobalFreeloaders.com  is an online hospitality network. As of December 2005, it has over 30,000 members. Australia is especially well represented.
Servas  was created in 1949 by Bob Luitweiler, an American who lived in Denmark. The organization spread rapidly all over the world and has thousands of hosts and travellers in more than 120 countries. Servas is also recognized by the United Nations.
Servas recommends applying for the program at least 4 weeks in advance of travel. Participation in Servas requires 2 letters of reference and paying a membership fee, which varies by country, and a personal interview with a local Servas coordinator. After the interview, the traveler gets a "letter of introduction" that's good for one year of travel, and a list of hosts in the countries they're visiting.
Travellers contact prospective hosts in advance (lead time varies as defined by each host), giving estimated dates of travel, and they may be asked to reconfirm one or two days in advance of visit. They can stay with hosts for up to 3 days and 2 nights, and are encouraged to stay the full time to develop a deeper relationship with the host.
Hosts provide sleeping space, sometimes this may be a guest room. Meals may be provided as well as assistance in visiting the city or area.
At the end of their trip, the Servas traveller is expected to provide a report to the local coordinator with updates to host lists (change of address and phone number, for example) and any other information that may be useful.
BeWelcome  is a three year old service founded in early 2007 by ex HC volunteers (also joined by several ex CS volunteers ) grouped in the non profit BeVolunteer organization. It is based on the BW rox open source project 
Membership and all features are free. New members' applications are checked before approval to avoid duplicated subscriptions and obviously fake profiles.
Travellers can then contact each other for accommodation requests with the internal mail system which protect email privacy. Spammers are removed as soon as they are confirmed when more than one member complain.
People inside BeWelcome can add comments about other people they know or meet, bad comments are also allowed in case it can be useful for other members.
This website also include a forum and classical hospitality exchange features (contacts managment, google maps, sophisticated search for host, links between profile, groups). In addition a member can have several version of his profile translated in various languages which can help him to find a host in a foreign country.
The Evergreen Bed & Breakfast Club is for people over 50 who enjoy traveling - members host each other in their own homes. Founded in 1982, there are over 4,000 members - mostly in the United States and Canada.
The Evergreen Bed & Breakfast Club members have developed into a network of over 2,500 delightful lodgings for considerate travelers by pooling their seldom-used guest rooms. The Club is a practical resource for travelers over 50 who enjoy people-to-people contacts. Members enjoy friendly hosts, comfortable accommodations in lovely homes, hearty breakfasts and firsthand information about local attractions.
There is an annual membership fee of $75 and guests pay their host a nominal gratuity of under $20/ night. Registration is online. Members receive online access where they can search for hosts in locations they want to travel to and interact with Google Maps to help plan trips. There is also a Member Lounge for blogs, managing one's account and profile and classified ads such as Home Exchanges. The Club has paid staff and an 800-phone number.
Belodged.com  is following the former globiwalk.com website which was launched in 1999, at a time as free accommodation networks have not been very well-known.
Unlike various open social networks, belodged.com does not try to set up a large mass of groups who are working for free for belodged.com. There is a professional team behind the platform that is supposed to guarantees the ongoing development of the website and its services.
Registration is anonymous and free, no personal data except of member's e-mail and geographic information is needed. With the anonymous messaging service in the member area other members can be contacted anonymously and at some point exchange information like e-mail, phone number etc.. belodged.com does not pass on any information - the system doesn't have them! Statistics can be found on the welcome page. The list of features is changing and growing as much the community is requesting.
Tripping  is a global community of travelers that was founded in 2009 and - just weeks after launching - grew to 500+ members from 40 countries. According to the website, Tripping aims to become the safest and best hospitality exchange in the world by offering increased safety and the latest travel tools and technology.
Like other hospitality sites, Tripping is a platform where travelers can meet local people. Travelers can get advice from locals in the Community forums or meet them in real life, such as for a cup of coffee or even a homestay.
Tripping offers advanced tools that are not yet available on other hospitality networks. For example: In addition to features like inline editing and advanced search functionality, calendars automatically track when and where travelers are being hosted.
Based in San Francisco, Tripping was founded by Jen O'Neal  and Nate Weisiger , who were two of the first employees at StubHub.com (a startup that was acquired by eBay for $307M in 2007). The founders also have experience in the hospitality realm, as Jen O'Neal is a longtime member of CouchSurfing and has hosted over 150 travelers across various hospitality sites.
Tripping is free for everyone and their motto is "For Travelers, Not Tourists"
Affordable Travel Club
The Affordable Travel Club  is a bed and breakfast hospitality exchange club founded in 1992. It is for people over the age of forty. Members pay an annual membership fee and may receive a printed directory or access to an online directory. Traveling members pay a small gratuity to their hosts. ATC is a community-oriented club with regular get-togethers. It is supported by staff.
Hospitality Exchange had its roots in the Travelers directory (now defunct) one of the originators of this concept over 40 years ago. Lower membership but passionate about its goals. Website, , explains it all, allows members to search for other members. $20/year to defray expenses.
TravelHoo was one of the oldest web-based hospitality exchange organizations, operating since 1997. In December 2005 there were more than 6,000 members in 114 countries, Eastern Europe and Asia being well represented. Today this network seems to not exist anymore . Only its online discussion group  is still reachable via different search engines.
The Pasporta Servo  ("passport service") is a home stay network for speakers of Esperanto, an international auxiliary language. It's sponsored by TEJO, the World Organization of Young Esperantists, who publish a book each year listing thousands of hosts in 80 countries.
Travelers pay a fee for the yearly host list. Hosts ask no fee for rooms, but each sets their own requirements for duration, number of visitors, contact ahead of time, and whether or not food is offered. Some hosts ask for compensation for food. Hosts receive the host list for free.
All travelers are expected to communicate with their hosts in Esperanto. Coordination with the service is in Esperanto, and the host list is in Esperanto. If you don't speak Esperanto, aren't really interested, and can't see learning a new language just to get into a home stay network, Pasporta Servo is not for you.
WarmShowers is an online hospitality exchange organization for touring cyclists. It has 10824 active members (data from 2009/12/04). It has been founded in 1993 in Canada, it has become an online organization in 2005. Its members are dispersed all over the world, although North America is still best represented.
Casa Casa  is a bed and breakfast hospitality exchange club loosely affiated with the Affordable Travel Club. It is for people age twenty four and over. Members pay an annual membership fee of $50, and their names and addresses are verified through the dues-paying process. Casa Casa's online membership directory is secure and searchable. Traveling members pay a gratuity of $15 to their hosts to offset the costs of providing breakfast. Casa Casa is supported by a full-time staffperson who is available to provide some assistance in arranging homestays.
Hospex  was the first Internet based hospitality network. Started back in 1992, after several years it converted to Hospitality Club:
Staydu.com  was created by two German students who had the vision that everybody should be able to travel without spending a lot of money. Staydu.com has gone online in September 2011 and already had more than 1000 registered users by the end of 2011. Everyone can use staydu, but the focus is on low-budget traveler who are looking for a long term stay. Hosts can offer their accommodation for three categories: for work, for money or for free. Staydu started totally for free, but to keep the advertisement low-budget staydu has that a limit of $ 200 / month for the category "money" and a limit of $50 of additional fees for the category "work".
Although a slightly different concept, home exchange is closely related to hospitality exchange, and there are a number of other agencies specifically for those who are interested in swapping homes.