At Xcoders last night, we got a presentation from John Chaffee of BusyMac on a new app they've been working on, and just entered public beta: BusySync.
From their website: "BusySync lets you share iCal calendars with family and coworkers on a local area network without a dedicated server and with full read/write access."
It was neat the way they did it, using Bonjour to discover calendars on machines around you (or your home). John would update a calendar on his machine, and a couple of seconds later it would pop up on his wife's computer. It handled conflicts, offline situations, and has options to add passwords for read and write access to calendars. And it was all very mac-like, just a couple of clicks and it worked.
These are the guys who originally created Now Up-to-Date (way back when cooperative multitasking was cool), so I expect to see a decent following with this app as well.