iCalShare is the largest directory of shareable calendars on the web.
Calendars are free to download and can be used with such applications as Apple's iCal and Mozilla Calendar.
Now listing 1560 calendars, in 35 different categories!
Upcoming.org is a collaborative event calendar, completely driven by people like you. Enter in the events you're attending, comment on events entered by others, and syndicate event listings to your own weblog.
As Upcoming.org learns more about the events you enjoy, it will suggest new events you never would have heard about.