If you have not heard about Etherpad there briefly it basically features online word processor, letter by letter collaboration, color coded edits when multiple users are editing the pad, infinite undo, history playback, ability to import and export HTML, Word and PDF files. While the foundation has recently launched Etherpad Lite there is another version which is Etherpad Full that also has Public/Private pads, Pad management, user account encryption and administration, image support and more.
The latest version Etherpad Lite 1.1.1 supports latest Node.js 0.8 server framework along with some control API's to know last edited by, last pads of author, authors on a pad, list of authors of a pad and others. The notable additions though are also Postgres support, better Microsoft Windows support and Unified timeslider and editing protocol.
Etherpad boasts Berkley, MIT, Mozilla and various other organizations run Etherpad and enables social media integration. While you can get hang of Etherpad should you wish to try on various public sites here is site that you can try its public implementation on - Pirate Pad.
While Etherpad Lite is completely re-written without some features that are part of original Etherpad, the foundation has left it to the community to decide weather both versions have to be supported or Etherpad Lite replaces Etherpad. The Etherpad Foundation maintains a public Etherpad Lite sandbox deployment for you to get started with. The public API key is EtherpadFTW. You can download Etherpad Lite from this location.