(By the way, you should have posted this in
iSync/CalDAV - Zimbra :: Forums)
Yes. With MacOS 10.5 and 10.6, though, its function is basically limited to syncing contacts.
You may not even need it for that, if you can get CardDAV to work with 10.6 and ZCS 6.0.7. But there are still a number of wrinkles in CardDAV to iron out.
E.g.
Bug 46458 - CardDAV do not work on the non-default domains in Zimbra and
Bug 46359 - contact groups dont sync via cardDAV
The iSync connector also sets up a CalDAV account for you in iCal, but that's not really necessary--you can do it yourself.
With some earlier versions of MacOS, I think the iSync connector will still sync iCal using non-CalDAV method. And if I recall correctly, some old versions of the iSync connector will sync Entourage, but that was quite a while ago--you'd have to search the forums for the full story on that.