In case you aren't paying attention to the news from Apple Expo Paris, and haven't hit Apple's page yet today, iCal was released today. iCal requires 10. and a mac.com account if you wish to share calendars.
I haven't had time to do much more than install it, so no comments as of yet.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020910062352255