Volunteering whilst travelling is a great way to 'make a difference' but it's not just about giving. Living and volunteering in a foreign country is a great way to get to know a different culture, meet new people, learn about yourself, get a sense of perspective and even gain new skills.
If you're only on a short trip, take time to visit a hospital, community project, etc. Those with more time can contact local NGOs, tourist offices, embassies, etc to inquire about longer term possibilities.
There are several ways to combine travelling and voluntary work:
In Europe, young European citizens (legal inhabitants, you need at least a residence permit) can participate in the EVS program ("European Voluntary Service") , which allows you to work abroad for 3 months up to a year, funded by the European Union at some kind of charitable organization.
If you would like to combine travelling and working on an organic farm, the WWOOF-ing - Willing Workers on Organic Farms  - might be something for you. You can be sowing seed, making compost, gardening, planting, cutting wood, weeding, making mud-bricks, harvesting, fencing, building, packing, milking and feeding livestock any of 80 countries.
Another organization with opportunities beyond the scope of just organics is HelpX .
Workaway  has a large database of options for those wishing to travel and volunteer in exchange for lodging and food. They have a worldwide selection of hosts from over 100 countries with a wide variety of opportunities including gardening, babysitting, art projects, language learning and animal care.
Volunteers Base  is a website where users can add volunteer opportunities themselves, with different kinds of opportunities.
If you would like to work in an international work camp for two weeks as an alternative holiday, contact your national branch of SCI .
If you pay US or Canadian income taxes, you may be able to take a charitable deduction for some or all of your trip expenses. The IRS is very strict on which organizations' trips qualify, and if you combine volunteer work with vacation, you will have to prorate the deduction on your airfare.
Various agencies of Western governments send volunteers abroad: the US Peace Corps , British VSO , Canadian CUSO  and so on. These are among the best volunteer jobs. All expenses — immunizations, travel, lodging, etc. — are normally covered and there is support — training, medical insurance, emergency evacuation if needed — and some sort of small salary. When you come back, these organizations look good on a resume. On the other hand, these positions are harder to get and usually require a heavier commitment, typically two years.
As an alternative, or to complement the working holiday, many people have chosen to embark on a volunteering experience. These meaningful experiences allow individuals the chance to give back to the international community, whether this be teaching in underprivileged schools, working with animals in Africa, or helping with community development project. Typically there will be a fee to participate in these programs, and participants are not paid. Most famously, Prince Harry embarked on a volunteering Gap Year in 2004 when he went to South Africa.
There are many organisations offering gap year placements. A few of these are:
Frontier Conservation  specialises in scientific and conservation projects in locations such as Tanzania, Costa Rica, Cambodia, Fiji and Madagascar.
Global Vision International is an award winning volunteer abroad organization running projects all over the world.
Gap Year Diver , specialists in gap year and career break scuba diving trips. Marine Conservation initiatives, learn to dive, become a Divemaster, teach in local schools, community work, Instructor training, language learning and much more.
The Mathieson Music Trust. You can help teach at a trust run school on the outskirts of Kolkata that gives deprived indian children a rounded education and a future through music. Here you also get the chance to learn indian music.
Rainbow Voluntours India  Join in Kerala or Kolkata and enjoy responsible volunteering and bespoke tours that give you an experience of India. Its more than just volunteering and Rainbow provides a perfect balance of living in India like a local and contributing to community projects that are hand picked and vetted by their experienced team in India. You can teach in local schools, help orphans, disabled children, women empowerment, education and literacy and even school renovations. Check www.rainbowvoluntours.co.uk for more details
The Great Projects . Beginning life as the multi award winning 'The Great Orangutan Project', the Great Projects now offers a wide collection of projects, used to showcase working models of animal conservation tourism around the world and promotes the importance of setting standards in endangered animal welfare and conservation in the tourism sector.
Yomps, Gap year, career break and adventure specialist, from volunteering in Africa to learning to surf in Ecuador, Yomps has it all.
Xtreme Gap  The adventure travel and extreme sports gap year company
Global Vision International , Specialists in gap year expeditions such as marine expeditions in the Indian Ocean and exploration in Patagonia. GVI also has over 150 volunteering projects around the world.
Gap Medics , Specifically aimed at students looking to go to medical or nursing school - Work experience and volunteer opportunities in The Caribbean, India and Tanzania.
Africa & Asia Venture (AV)  - a founding member of the Year Out Group , offering 5 week to 5 month group volunteer projects in the developing world with 24/7 support throughout.
RealGap UK  RealGap DE , Specialists in gap years, career breaks & adventure travel holidays. RealGap are experts in providing unique & exciting volunteer projects, working holiday programmes & backpacking packages across the world.
VentureCo Worldwide , specialists in South America Gap Years. Combining language courses, volunteer projects and adventure travel.
Raleigh  Youth and education charity offering challenging enviromental, community and adventure programmes for 17-75 year olds in Borneo, Costa Rica & Nicaragua and India
Projects Abroad , Global organiser of overseas voluntary work placements. Projects Abroad runs placements like Teaching, Care, Conservation & Environment, Medicine & Healthcare, Sports, Culture & Community projects and Journalism.
Volunteering Solutions  International Volunteer Programs Abroad. Volunteering Solutions provide volunteering placement in different projects such as teaching, orphanage, conservation, medicine and healthcare, cultural exchange programs as well as internship opportunities.
EDGE of AFRICA , Small organization that offers volunteer and internship placements in Southern Africa. Conservation, community and sports volunteer placements available as well as family and tailor made positions.
VAE Kenya , a small UK organisation that organises challenging teaching placements in rural Kenyan schools.
Rainbow Voluntours  offers some truely rewarding, unique and enjoyabl volunteer opportunities in India.You can work in educating slum children, orphans, disabled children, helping youth and women from disavantaged communities. They assist grass root organisations and local initiatives in India
Skills Venture , a new company that runs short sabbaticals for US and UK business professionals, spent providing mentoring and consultancy support to entrepreneurs in Kenya.
Cross-Cultural Solutions(CCS) is a non-profit organization that offers 1-12 week volunteer abroad programs in 12 countries, where you work side-by-side with local people, stay in a CCS Home-Base, and learn first-hand about local customs by participating in Cultural & Learning Activities.
Praktikawelten  offers more than 300 volunteer projects in 17 countries as well as work and travel programs in Australia, New Zealand and Canada.
There are also various organizations that recruit volunteers. These may not cover the major expenses such as airfares and travel insurance. Some charge a fee for placement. A few of these are:
Global Nomadic offer serious and professional placements around the world, all have been personally visited. 
Treetop Country offers affordable and unique volunteer/living opportunities in a north Thailand village.