Citadel is a turnkey open-source solution for e-mail and collaboration. One simple installation delivers a multitude of powerful features, including: e-mail, calendaring/scheduling, address books, bulletin boards, mailing list server, instant messaging, multiple domain support, and modern 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.