Google has released a beta version (no surprise there) of the much anticipated, AJAX-based Google Calendar. GoogCal is partially tied to your GMail as it will recognize dates in email messages and offer to add them to your calendar, and it can also invite your GMail contacts to start a Google Calendar. Of course, GMail is not required to use GoogCal, all you need is a Google Account. — If you need/want a GMail account, give me a yell, I can invite you.
C|Net News has a article about the new app. In their article they state that like Google Maps, and many other Google services, an API will be available for Google Calendar. What’s even better is the Calendar will be able to sync up with Outlook and mobile devices.
Search Engine Watch says “Google Calendar makes an impressive debut.” The SEW article has a bit more information than the above CNet article. 
Sadly, but to no surprise, GoogCal doesn’t work with Opera (yet). I’ve tested both the 8.5 and the latest 9.0 build of Opera and just get a blank screen. I’m sure Google and Opera are working to resolve the issue as quickly as possible.