Currently, Ventrilo is one of the most popular voice communication systems for multiplayer games. The application uses VoIP technology, the same as Skype.
Surround sound and real time special effects are the best parts of using Ventrilo, which supports multiple conversations (between various users at the same time) or private conversations (between two users). If you don't want to participate, you can also just listen to what everyone else is saying. Access to the Ventrilo server and its channels can be password-protected.
The application that we have here is a Ventrilo client. You'll find the server on the program's official webpage. The client is available for the following platforms: Windows, Linux, Solaris, FreeBSD, NetBSD, and Mac OS X.