What is a good freeware voice chat program?

I want to be able to chat with my friend while we game online together by having a program that will work simultaneous with the game in the background while I play.  This way we can voice chat and communicate much faster than stopping to type out messages and possibly suffering horrible anal raping from the vicious computer AI who cannot wait a few seconds while I finish my conversation.  Does anyone know of a good program that can pull this off effectively?