This is pretty hot right now… supposedly the definitive Web 2.0 calendar. Check out 30 Boxes. They just opened up their beta, and it looks pretty neat.
The idea is that it’s a collaborative calendar. You set your appointments, and can share some/all/none of your appointments with everyone/friends only/no one. Of course, since it’s very Web 2.0, the whole thing is in AJAX, and it supports tagging (which is nice)
You may have noticed the Calendar section on the right. You can place this HTML/Javascript badge on your site, and allow the public to see what you want them to see, and whether or not your busy.
I think it still has a ways to go, but definitely has potential. Check it out, and let me know what you think.