OpenGroupware website
The website for OpenGroupware is of course:
http://www.opengroupware.org/
You will see some screenshots there of OpenGroupware working with Apple iCal. Since the project relies on Cyrus there is nothing preventing its use on Mac OS X Server 10.3.
Regarding Mac OS X installation, their FAQ as this to say:
[quote:a2a228570c]Most of the core libraries already work on MacOS X. Hopefully someone will step in and complete a MacOS X port (if you are interested, write an email to feedback@opengroupware.org.[/quote:a2a228570c]