Sounds crazy, but many places you find on the net charge for you to volunteer. In my opinion, volunteering should be free. Here are a couple of really good sites to find free volunteering opportunities, enabling you to learn new skills and culture, and for organisations to get help in return. Sign-up is free and their opportunities are found all over the world.
Can’t decide? A Lonely Planet blog thread is found HERE debating the pro’s and cons of each website.