Mumble is a free, open-source, cross-platform voice over IP application. Its primary users are gamers, and it is similar to programs such as TeamSpeak and Ventrilo. It uses a client-server architecture where users who want to talk connect to the same server.
Mumble has a very simple administrative interface and most of the engineering effort is put into sound quality and low latency.- Source http://en.wikipedia.org/wiki/Mumble_(software)
You can Download Mumble from here: Download Ventrilo