I can heartily recommend One, Two… Tree! as a well-structured and well-intentioned organization. Some background: They place volunteers in the public school classrooms – alongside the local, licensed teachers – to provide instruction in English as a foreign language.
Working with One, two…tree! was one of the best months of my life thus far. I grew and stretched in ways that I would have never through possible, and I made friends that I still keep in touch with today.