Upcoming.org is a social event calendar, completely driven by people like you. Manage your events, share events with friends and family, and syndicate your calendar to your own site
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.
12 Top, Free Ways to Collaborate Online February 28th, 2008 (4:00pm) Samuel Dean 22 Comments Applications that make it easy to share and collaborate are often of much more use to web workers than they are to standard office workers. Especially if you wor
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
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