About BusySync
Share iCal calendars with other users on your LAN without the need for a dedicated server, as well as sync iCal with Google Calendar. Shared calendars can be viewed and edited by multiple users and changes are instantly synchronized with all users on the network and/or with Google Calendar. You can even make changes to shared calendars while offline and your changes will sync when you reconnect to the network.

BusySync is a Mac OS X System Preference Pane that runs in the background and adds calendar sharing and syncing capabilities to iCal.

Read more: http://www.apple.com/downloads/macosx/productivity_tools/busysync.html