I haven't been on one (yet), but I've done a lot of research. I found a really good way to see what was out there was to search on www.goabroad.com. They have a whole section devoted to international volunteering.
Another good resource is the UN volunteering organization resources. Or look on the Bill and Melinda Gates foundation website. They have a really good list of international volunteering organizations.
Keep us updated on what you find/decide to do! How long are you planning on going for? If you can do 2-3 months, I would suggest www.worldteach.org. I'm actually planning on doing one of their programs. Haven't decided on the year or the short term program yet.
I just noticed this old post from Daily Candy about volunteer vacations -- if you're still researching the opportunities, it may have some good links! http://www.dailycandy.com/article.jsp?ArticleId=28475&city=15