Citadel

Citadel is an open source groupware server. While others focus on automating business processes, Citadel focuses on connecting communities of people together. Users love Citadel because it's software that helps them work, play, stay in touch... without calling attention to itself.

One of the advantages to Citadel is that all of its data stores are built in; it does not require external IMAP and MySQL servers. Because of this, Citadel is very easy to install.

= External links =
 * Citadel web site