TeamSpeak

A VoIP application.

http://www.goteamspeak.com/news.php

teamspeak is not open source.

The difference to most other of its kind:
 * teamspeak can be configured to good old push-to-talk, known from citizen band. You only hear what others want to make you hear. It is therefore much less dominant in combination with a collaborative editor session. When nobody pushes the button there is silence.