I haven't done it myself, but I know there are many possibilities. First, there's lots of websites you can check for classic global volunteering opportunities. One of the hot things right now is WWOOF, which is volunteering on organic farms. Or there are zillions of English-teaching jobs all over, especially in Asia. That's not volunteering, but you get paid very little so it might as well be. I've just spent 4 months traveling in Asia so if you want some specifics especially about China let me know.
