Mumble is a voice chat application for groups. It has low latency and superb voice quality. While it can be used for any kind of activity, it is primarily intended for gaming. It features an ingame overlay for DX9 and OpenGL based games, showing clearly who is talking even for fullscreen applications.
The server process (called murmur), runs on most moderns OSes. It has secure communication, and is fully controllable through D-Bus.