Mumble is a voice chat application that lets you communicate with fellow players over the Internet. It also features a text chat with text-to-speech capabilities. The software consists of a client (Mumble) and a server (Murmur), this way the communication channel is open all the time and players can freely join and leave in contrary to peer-to-peer software, e.g. skype.
Mumble is not that widespread and few commercial servers are available but the user base is constantly growing. The sound quality and the latency is good and configuring the servers, permissions and channels, is easily done within the application.