Leave it to Google. There have been plenty of web-based calendars out there for a long time now (Yahoo, various Web 2.0 ones, etc.), but when Google gets into calendaring, it becomes a hot commodity. Google's interface is classic Google: simple, powerful, elegant. And Outlook syncing is in the future, so it may become a replacement for the Microsoft powerhouse.
One of the more intriguing features is the open standards the Google Calendar is built on: iCal. Used by Mac users since the advent of OSX, it allows the web based sharing of calendars. What kind of calendars, you ask? How about every kind you can think of?
If I could get it to sync with Salesforce, the Palm, and Outlook, I'd be a convert. You listening, Googlers?