EtherPad is a collaborative, web-based text editor that supports concurrent document changes, versioning, and built-in formatting for a group of people. Etherpad makes it easy for people to simultaneously type, in real-time, on one document. Each author enters their name and selects a color - as they type, text shows up as a color. A collaborative "pad" can be created by anyone, and each pad has its own URL. It is possible to set up password protected pads. It is free and open source software that can be installed on a web server, and there are public websites that provide the EtherPad application as a service.
Google has acquired EtherPad, and the program has been made available on Google Code: http://code.google.com/p/etherpad/. Installation requires Java, Scala, and MySQL.
Access some Etherpads here: