Some people may not be able or willing to spend much money, but wish to see the world anyhow. It is possible to travel with very little or even no money at all.
This means either keeping expenses low or earning money while one travels. Important: in your quest to reduce expenses, do not steal or mooch from others. There is honor in ultra-budget travel.
If you have more time than money available, you will find out that walking or cycling is possibly the best way to discover a new place. This can be combined with hitchhiking, which is faster and normally just as cheap.
Use public transport instead of taxis or other faster, more expensive means of transportation. Some places are experimenting with free public transport (such as Hasselt in Belgium and Portland (Oregon)). However, in some places, it can be cheaper for three or four people to share a taxi than to take public transport. Shop around and compare.
For public transport, look into multi-use tickets. Many systems have tickets that can be used a certain number of times, or over a certain time period, for a considerable discount over buying each ticket individually.
Even national and international rail and bus networks may have discount tickets for a month's or several months' travel. You should also check what discounts you're eligible for: Western Europe frequently has blanket discount schemes for people under 26, Great Britain has a youth discount card that you can buy and which pays for itself after three or four journeys, and many countries have discount schemes for students, pensioners and sometimes disabled people.
In Germany you can try to hitchhike on trains with people carrying Schoenes Wochenende tickets on the weekends.
In the United States, you can try a driveaway car service. In this service, you pay a small fee to an agency to deliver a car to a business or individual in another city, often hundreds or thousands of miles away. The customer wins by reducing his shipping costs, and you win by having a car to drive for cheap. You are often allowed a certain amount of flexibility in the route and delivery time, allowing you freedom to explore.
Camping is an obvious choice for cheap accommodation, and it's often the closest accommodation to lots of natural attractions. This will mean burdening yourself with whatever camping equipment is necessary to protect you from the weather at your destination. Also, many popular sites like national parks limit camping to particular spots and have you pay for a site. This is almost always cheaper than hostels except in very very popular camping spots.
You could sleep rough, that is sleep out of doors wherever you find a spot. This is difficult for three reasons: the first is that it will often get you in trouble with the police if you do it in urban areas; the second is that it makes you unusually vulnerable to crime, both theft and violence; and the third is weather. There are few places where this is seen as an acceptable way to holiday (ie "sorry officer I'm on holidays" is unlikely to be believed): however in Japan you can participate in the nojuku tradition.
The objective of hospitality exchange networks is to meet new, and local, people. It can be a great way to get a free place to stay the night, but besides that it's a fun and easy way to get acquainted with an area, city or culture. Active users of online hospitality exchange networks also tend to have broadband connections, which you can use while you are staying there.
You can stay in hostels or guesthouses, usually the cheapest type of commercial tourist accommodation. Many hostels offer cheap one- to four-person rooms, but the cheapest of all are dorms shared by up to twenty people: you'll usually be given a key to the room and left to choose a bunk bed. There are some international hostel associations, members of which get discounts at participating hostels. Bring a sleep sheet (two sheets sewn together like a sleeping bag) so you will not have to rent linen at hostels. Dorms are a great way to meet fellow travellers.
The cheapest places to buy food are traditional markets, supermarkets and street vendors. In countries with peculiarly high hygiene standards, you may be able to find perfectly acceptable food in supermarket's rubbish.
In some cities there are very cheap restaurants in squats, usually selling vegeterian or vegan food for the price of the ingredients; for example Berlin's Volkskuechens. Some countries also have heavily subsidised university restaurants sometimes open to foreign students as well. Germany for instance has Mensas, offering famously tasteless meals for 2-3 euros.
Self catering, buying your own ingredients and preparing your own meals, is a great way to stay on a budget. Many hostels provide kitchens where you can cook your meals.
For restaurants, avoid eating in the main tourist thoroughfares. If you get into the side streets and back alleys, you'll find cheaper restaurants that often serve tastier, more authentic meals.
Many art galleries, museums and other attractions are free. Of those that require an entry fee, some have discounted or free days at least once a month, or a time after which admission is discounted or free. Tourist information offices will sometimes be able to tell you about these.
The most straightforward way to earn money on the road is obviously to find some work. This is more easily done through contacts, and as a matter of fact hitchhiking may come very handy here. Contacting expatriates may also provide opportunities.
Obvious jobs for travellers include harvesting, teaching English and waiting at restaurants or bars in tourist areas.
Wwoofing is a term for working 5 hours or so a day in exchange for lodging and food: it stands for World-Wide Opportunities on Organic Farms. See wwoof.org.
Resources exist, such as Susan Griffith's biennial Work your Way around the World published by Vacation Work.
In many places in Western Europe, and possibly also in other parts of the world, you can find give-away shops, shops where you can take things you want (as long as you don't take too much), and where you can leave stuff you don't need anymore.
BookCrossing is a book exchange network. Books are travelling through the world, looking for people to read them! You might have encountered books with BookCrossing stickers already, but on the website you can look for places to find them.
Tips and tricks
To really stretch your budget, here are some money-saving tips: