Primarily a research and review platform with a database of more than 17,000 overseas programs, it covers volunteering as well as studying, teaching, internships, gap years and other experiences. Volunteer listings can be filtered by destination, type, length and other criteria, while tens of thousands of participant reviews and thousands of interviews provide a valuable reality check before booking. Particularly useful are ratings for such factors as impact, support, value and safety. Unlike a provider promoting only its own trips, this site lets travelers compare competing organizations and then investigate the individual operator before committing money.

Read more in my post 5 of the Top Websites for Voluntourism.