CalDav (Score 5, Informative)

CalDav is the wave of the future, with most calendaring clients supporting it (but not MS), and many servers commercial and otherwise also supporting it (Zimbra). The real coming out party was the commercial release of both OSX Server 10.5 and the client, which have both ends. But guess what, the server is open source: calendar server can be gotten and put on any platform. If you want something today, Zimbra or OSX Server are there for the taking. RedHat has a Messenging product coming out based on Zimbra for this exact purpose.