In 2007, with a colleague, I set up SE7EN – a not-for-profit organisation that lists all those small and inexpensive volunteer projects that are so hard to find when you're browsing the 'net.
SE7EN lists free and low-cost volunteering opportunities on social and environmental projects all over the world - schools, orphanages, wildlife reserves, natural parks, sustainable living initiatives and more.
Since it was set up, over 10,000 volunteers and 'host projects' have registered with us. As of July 2012, we are listing over 540 volunteering opportunities in 123 countries, and have 4000 current active volunteers.
Through SE7EN, you can find yourself teaching English in Nepal, mentoring street children in Cameroon, protecting Andean bears in Ecuador, establishing a Permaculture farm in Palestine, or any number of other things!
Because you arrange your volunteering yourself directly, it's easy to combine it with independent travel or backpacking. And you'll often receive accommodation and food in return for your volunteering, helping to reduce the cost of your travelling while making a positive impact on the countries you visit.