Apple’s Darwin Calendar Server (DCS) is a free, open-source CalDAV server that works with iCal. It’s not the only one, but it’s a good choice—as long as you don’t mind a bit of command-line tinkering.
OS X Server 10.5 includes iCal Server, a full-featured yet easy-to-configure CalDAV server. Beyond the ordinary requirements for OS X Server, though, iCal Server has some additional qualifications.