Citadel is a turnkey open-source solution for email and collaboration. One simple installation delivers a multitude of powerful features, including: email, calendaring/scheduling, address books, bulletin boards, mailing list server, instant messaging, multiple domain support, modern AJAX-style web interface
Chandler Project is an open source, standards-based personal information manager (PIM) built around small group collaboration and a core set of information management workflows modelled on Inbox usage patterns and David Allen's GTD methodology.