Mashable has a great roundup of online calendars and calendar resources that are available right now. The usual suspects are there along with some lesser-known calendars. Of course we all know who the leader is still (at least until Yahoo releases their
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.
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