The online calendar that looks most promising is Kiko. There is also Planzo, HipCal, and CalendarHub though.
The reasons that I might want to use someone else's calendar software is similar to why I'm using someone else's book catalog. It can be useful to share things and there will likely be software improvements that I won't have time for.
Any thoughts on such?
Update: I'm using Google Calendar primarily because it's linked to my google e-mail account.