I'm no expert in calendar applications, but I'm very tempted to say, this must be the most brilliant web-based calendar I've seen: Hula web interface. Drag-drop date/time, duration, faded borders, in-place editing.. brilliant!
Hula Project, btw, is
a calendar and mail server whose goal is to be fun and easy to use, while scaling effortlessly from small groups to large organizations with thousands of members.
Hula is an open source project led by Novell