Note: As of 19 June 2013, Spanning Sync service went offline and sales have permanently ceased. The developer recommends using the uninstaller provided in the download to remove the preference pane.
Spanning Sync... Synchronize Google Calendar and Apple iCal.
Save $5 on Spanning Sync through this discount link! Spanning Sync costs $25/year or $65/forever.
Share calendars between multiple Macs. Share calendars with your coworkers, family, and friends. And while you're at it, connect Google Calendar to your iPod, mobile phone, and other devices.