Mumble is a voice chat similar to Teamspeak. It is suitable for gaming, but can be used for general chatting too. Mumble uses a centralized server an desktop client applications that connect to this server. The clients are avaiable for many platforms.
apt-get install mumble
On Debian based systems install the server like this:
aptitude install mumble-server dpkg-reconfigure mumble-server
A wizard will guide you through the configuration:
Mumble needs both TCP and UDP on port 64738 accessible.