I just tried out kiko, an ajax calendar service. It looks kinda cool, but it failed to import my calendar data ("invalid file") and contacts (spun forever). Update: they fixed the problems I reported the same day, and they seem to have most of the relevant features I want planned. See comments for details. Hmm... reminders by IM is kinda interesting. But I'm not sure I could rely on a remote service for that; it's critical that I be able to create/change appointments and get reminders in the middle of the day, even if I have no GPRS. I reported the import problems and sent a feature request for subscribe. hCalendar/hCard support would be cool too.