I first talked about calendars in my post Tada Calendar back in March 2005. It seems that a few other people got the same idea probably around the same time, with the recent announcements from 30boxes and SpongeCell.
One thing I noticed in all these new calendar apps is the heavy use of popup dialogs. I just don't believe dialog windows have any place in future web apps. The nature of the web as a page within a browser calls for a different interface than a desktop. I don't think simply mimicking a desktop calendar is the right approach. Gmail manages to be better than a desktop email client without any dialogs. I think with some creative ideas, the same concepts can be applied to a calendar.