Finally Google Calendar is out which makes perfect use of AJAX.
What separates "GCal" from other calendar user interface (UI) is the dynamic date range for all views.
Another plus would be the SMS based alerts and the fact that its integrated with GMail, the alerts went further than just a scheduled alerts. Whenever an attendee accepts or rejects an event, GCal also send alerts whenever someone accepts or cancels an event. The integrated GMap is also interesting, its great when holding parties and making sure your guests wont get lost.
I would be keeping a close eye on GCal, I'm sure there will be a lot of mash-ups services comming out that uses its iCal and XML features.