BusySync is a Mac OS X System Preference Pane that runs in the background and adds calendar sharing capabilities to iCal. Leveraging Apple Sync Services and Bonjour, calendar events created by one user are instantly published and synchronized with other iCal users on the network. Users can share and edit calendars, make changes on each others calendars, and everything is automatically synchronized between each user, on each Mac.
I think BusySync is a huge deal. Leopard’s new iCal sharing is very cool, but requires a dedicated server. For a household or very small business or partnership, BusySync’s server-less “just install it on your Mac” model is perfect.