BusySync enables you to share iCal calendars with others on your local area network or over the internet. It does not require a dedicated server, and calendars may be password protected to allow read-only or read/write access.
BusySync must be running on each computer you wish to share calendars with. BusySync installs as a System Preference Pane and runs in the background keeping calendars in sync over the network. If you go offline, you can continue editing calendars, and when you reconnect to the network your changes will be synchronized with others.
The next version expected in February 2008 will sync with Google Calendar