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:

• Autostart mumble-server on server boot? – Usually this is what you want.
• Allow mumble-server to use higher-priority ? – Voice chat is a real time thing, so if you don't have other mission critical services on this server answering “yes” might be an option.
• You definitely want to set a very safe password for the SuperUser account!

Mumble needs both TCP and UDP on port 64738 accessible.