Google released the beta version of its Online Calendar last week. You need a Google account to access it and as far as I can see - it looks sleek.
Yahoo My Calendar and Microsoft's Live Calendar are both slower and more cumbersome to use than Google's. GCalendar appears to use some Web 2.0-esque trick to speed up the create events and generally how you go moving around; there's no apparent screen refresh and everything is as intuitive and easy to use as you would expect from the search giant.
Creating events and changing views only takes one click and no waiting time. Although the interface is simple, you can create several calendars, make some of them public, do repeats and invite guests. I also particularly liked the Agenda and the ability to import information in CSV format (i.e. from MS Outlook) The only drawbacks that one may see is the fact that Google Calendar (as are so many other Google services) are still in Beta. However, as a regular user of Microsoft Outlook and a tester of so many online calendars, Google Calendar still comes out tops to me.