Mumble is an open source, low-latency, high quality voice chat software primarily intended for use while gaming.
Mumble is a voice chat application for groups. While it can be used for any kind of activity, it is primarily intended for gaming. It can be compared to programs like Ventrilo or TeamSpeak. People tend to simplify things, so when they talk about Mumble they either talk about "Mumble" the client application or about "Mumble & Murmur" the whole voice chat application suite.
The client, Mumble, runs on Windows, Mac OS X and Linux.
The server component, Murmur, should run on anything you can compile Qt 4 on.