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.