Mozilla.orgis Open Source calendar program, called Sunbird, was updated to version 0.5 and released for Mac OS X on Thursday.
Sunbird, while developed by the Mozilla team, is designed to work independently from an e-mail program like Thunderbird. However it does support Google Calendar. There is another Mozilla project, Lightning, that handles meeting invitations via e-mail and integrates with Thunderbird.
Sunbird can import calendars from iCal because they use the same file format. The Mozilla Calendar FAQ explains the use of Sunbird, its features, and limitations. Some users may find Sunbird preferable to iCal in its layout, features, or functionality.
Sunbird 0.5 for Mac OS X requires Mac OS X 10.2.x or later. It is Open Source and free.