WEC International (U.S. Office)
P.O. Box 1707
Fort Washington, PA 19034
WEC (Worldwide Evangelization for Christ) is one of the largest interdenominational mission-sending agencies. It is active in over 80 countries and arranges short and longer-term mission trips.
WEC also helped produce an excellent 978 page book that covers every country from a Christian perspective. It is entitled Operation World, and the most recent edition was in 2010.