United Planet

United Planet seems like a well-organized program with a good, solid reputation. They have a wide variety of programs- there's pretty much something for everyone.

* solo travelers
* college/ grad students
* families
* corporate groups
* teens
etc.

40 countries to volunteer in - there's at least 1 program in every continent (except Antarctica!)
Including: Tibet, Ecuador, Cambodia, Romania, and Jordan.

Another unique feature they have are "group" trips organized and customized for groups.

* Social work related internships in Peru, Nepal, and Jordan!
* Spanish immersion