I've been playing with google calendar a bit of late. It's really very good. It's not 100% feature complete yet, but it's damned good for a beta. Of course, a gmail account comes in very handy, I can send an invite out upon request. (Update: The calendar page now has account creation on it... so this may not be needed).
There are several things missing from the calendar: 1) Outlook synchronisation (one can get the calendar into outlook, but it's not easy, and it's not 2 way) 2) Rationalising invites. E.g. If you have an account under login A, and I invite you to an event at email address B, login A cannot see the event. This is a good thing (security), but can be annoying at the same time. It's fixable by allowing users to link A and B, (e.g. test email/confirmation). 3) A simple way for anyone to view a public calendar (there are workarounds using things like iwebcal) 4) Notifications, and inclusion on Agenda for secondary calendars 5) Customisable and integrated 'To Do' lists. 6) With a lot of calendars or short events the pdf printouts get messy. 7) When an event spans midnight, it looks like an all day event rather than blocking out the busy times. 8) The ability to 'share' several subcalendars in one go (i.e. I have work and home on seperate calendars, my free/busy times combine them both)
With these things aside, it's not bad at all. It's nice to be able to see my events from whereever I am - and for now the one way download to outlook is fine (now I've got it going).
The utility goes up when friends and aquaintances have similar online calendars and share them too. When one shares under google calendar, there are the options to only allow a sharing of 'free/busy' times, or to allow full detail. I only ever share the 'free/busy' times unless it's a fully public calendar (see below), or I'm sharing a calendar with the wife! The site also provides ical feeds (useful for mac users) and xml feeds. There are 'private' feeds which are what you use to look at a private calendar from your desktop, the public feeds only work if the calendar is public. Obviously.
There are lots of public calendars on the system already. For those interested in the world cup, this calendar from Jez Nicholson could be useful! The calendar 'overlays' onto the view, and can be switched on and off at will, i.e. it won't get muddled in with your own events.