These mozilla calendar products (one stand-alone, the other an add-on for thunderbird email client) can also be used in combination with Google Calendar for those who want to use these Agenda applications (cross-platform).
I noticed this upon Googling following a request in the dutch Pardus forum.
(Note: I am personally not
a regular calendar/agenda user myself)
An english link is e.g.:http://www.linux.com/feature/118068