Mumble is a VOIP system composed of a client package and a server package that provides channels for groups to do voice communications together. It's designed to work very clearly with very low latency and can also work in very low bandwidth conditions if necessary. I've installed this on our server so we have a means of doing voice chat together without needing Skype which is not private and has also degraded a lot in quality having very high latency and breaks up a lot.
Installation was very simple on Debian involving an installation of the mumble-server package on the server side and the mumble package on the client side (on Ubuntu you'll need to add-apt-repository ppa:mumble/release and update before installing the mumble package). The server required practically no configuration (I pointed it to our web-server SSL certificates and kept everything else as default), and the client required only minimal configuration to calibrate the microphone and bandwidth conditions and to add our server to the list of available connections.