Seek free calendar groupware for mixed env.

Hi all.
Tis my first post here.

Can anybody recommend a free calendar groupware for mixed env. (i.e. Win 2K and Win XP clients in LAN or remote LAN with a RH Linux server as storage hub ?

The groupware would be located on the RH9.0 lx server and should be able to manage read/write requests by various clients simultaneously.
If it can be written in Java with a GNU license, great.

I am just beginning this little research, so don't assume I know anything about the subject.

Thank you in advance.