For some time now I've been playing with Google Calendar.
As other google products it has nice very intuitive interface, and of course is still in BETA ;).
At first I've found this calendar to be not very useful, apart from it's notification feature. Unfortunately for it to work your web browser must be running and you have to be logged onto google calendar. Moreover it's not very visible...
Recently my fiancée started to use google calendar. And then I realized that it became quite useful. I've subscribed to her calendar and Kasia subscribed to mine. Now I don't have to remember when she has her classes and so on. I always can look it up :D.
It has a lot more features that ease up making appointments with other people. But up to now I haven't had a chance to try them out :/
Today as you probably noticed I added my two calendars to my blog. So now everyone who's interested can check when I'm available... Of course I don't put all my appointments into this calendar ;)