I use Zimbra for my mail server and I am happy to see Zimbra now supports CalDAV. This means you now can sync your calendar for Mac OS X using Mail.app and iCal without and third-party add-ons.
To setup a CalDAV account in Mail.app:
- Go to Preferences > Accounts and click the "Plus" button to add a new account.
- Set any description, and enter your Zimbra username and password
- Click the disclosure triangle and enter the server address
Click Add and your calendars will immediately start synchronizing.