We currently provide binary packages for RPM and DEB based GNU/Linux distributions. The sources should work on any Intel based GNU/Linux without issues and they do work on SPARC/Solaris. Further we are currently in progress porting to the IBM iSeries platform.
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.
Regarding Windows, check our mission document. We are not actively working on it, but have it on the radar ;-)
Finally, the sources should compile without major issues on any Unix(tm) like operating system which is able to host GCC and GNU make. If you managed to build on your specific platform, please send us feedback.