I up-voted for Google Calendar + CalDAV as I use that for my personal calendars and it works well for me, but I don't use To Dos/Tasks at all so I don't miss that functionality.
I wouldn't suggest using Mac OS X Server's iCal Server (also CalDAV-powered) anyway as there are still a few odd bugs w/group calendars, permissions, re-sycing when disconnected, etc. Also, it really requires an Open Directory setup with the workstations bound and don't expect to be able to invite anyone to meetings if they aren't in Open Directory. Overall it's okay, but probably more work than you're looking for.
One other suggestion is BusyMac's BusySync. Shareware and does a great job of sharing & syncing calendars on a local network.