CADIP is a Canada-based group that organizes short and long term "workcamp" volunteer trips around the world. Their programs include environmental, social, and community-building activities. You must be at least eighteen to participate, and their fees vary by program. No specialized training is required, and there are several trips year roung to choose from.
For more infomation, visit http://www.cadip.org/index.html.
Global Citizen Journey is a nonprofit organization that focuses on a few select "journeys" to support their existing programs in Africa, as well as help develop future "journeys." They seek volunteers to both travel with the program as delegates and to stay in the United States and develop new projects.
For more information, visit http://www.globalcitizenjourney.org/index.asp.