Anarchogeek does a great review of Trumba, EvDB and Upcoming.org. The comment that caught my eye was:
"It’s [calendaring is] a hard problem, and i am not sure what the right solution is, but i know i haven’t seen it. It’s a very different problem that that of a PIM or a business group / team scheduling. The majority of calendar apps are PIM focused, which is not my personal interest, as my life is not full of business meetings."
This was one of the reasons for the putting forward RFC 2445 iCalendar Basic – it strips the standard of the PIM releated activites that are not interoperable including to do’s, journals, recurrance rules and timezones.