Did you know that the iCal online calendar functionality in Mac OS X can store your calendar on a WebDAV server? Not just Apple's server (which you have to pay to use now I think) but any WebDAV server.

Sadly, it's not compatible with the way Microsoft Exchange does calendaring over WebDAV. The calendaring working group at the IETF long ago rejected the idea of using WebDAV to do any calendar access interoperability despite obvious implementor support.