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
Users love Citadel because it’s software that helps them work, play, stay in touch... without calling attention to itself. System administrators love Citadel because it installs in minutes without the need to manually integrate all the different components together.The key to Citadel’s versatility is its unique architecture. A Citadel system is made up of containers called “rooms.” A room may be used as an email folder, a discussion forum, a real-time chat, a mailing list, a calendar, an address book, an RSS sink ... sometimes a combination of any of the above, and certainly any other uses which could be added in the future. Furthermore, you can replicate rooms between multiple Citadel nodes, allowing you to set up a federated, distributed messaging environment.With that in mind, Citadel excels at applications such as:
E-mail and groupware (shared calendaring, etc.)
Bulletin Board System (BBS)
Online discussion forums / chat rooms
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.