Citadel is a complete and feature-rich open source groupware platform.
  * Email, calendaring/scheduling, address books
  * Bulletin boards, mailing list server, instant messaging
  * Multiple domain support
  * An intuitive, attractive AJAX-style web interface
 .
 The Citadel system is extremely versatile. It provides numerous front ends to
 present to users, such as a text-based interface, an AJAX-style web interface,
 and many popular PIM clients using SMTP/POP/IMAP. All of these can be used
 simultaneously.
 .
 It's also extremely scalable. Not only can a well-equipped Citadel server
 support a large number of concurrent users, but you can also build
 a distributed network of Citadel nodes that share rooms and their content.