Google Calendar officially supports CalDAV protocol. CalDAV is a calendar standard protocol and format supported by Apple in iCal, Mozilla in Sunbird and Microsoft in Outlook. It is now possible to synchronize iCal and Google Calendar, but if basic 2-way synchronization should work as expected, not all features are currently supported. You need to be the owner of the calendar, shared calendars can not be synchronized yet.
Google developed a small application for making the synchronization setup a easy task: Calaboration. As a reminder, there is an alternative offering more features for synchronizing iCal and Google Calendar: Spanning Sync; it is not a freeware, but its price might be worse it for most of us.
[translation by
