Google Calendar came out a couple of weeks ago, and I've been having a bit of a play with it. I use Apple's iCal as my main calendar, syncing it with my phone so that I've got access to my diary on the move. However, I've also been trying out some of the available online calendars that have been cropping up recently to see if I could use them alongside (or perhaps, instead of) iCal.
A lot of online calendars have sprung up recently, and I've personally tried out 30boxes and Kiko. Both have a lot of nice features. 30boxes has a lovely stripped-down and clean interface, with a dialog box that floats over the calendar then disappears when not needed. However, it doesn't show blocked out sections of time in either the daily or weekly views, which I've come to depend on for judging how busy I'm likely to be on a given day. Kiko is a bit busier visually (though you can turn parts of the interface on or off), but shows blocks of time. Given the clout that Google has, both of these applications are going to have quite a fight on their hands.