I'm one of the developers at Thousand Parsec project. We are creating a free and open source framework for building custom 4X space strategy games. Servers and clients for it should also run on Mac OS. Currently there is one stable client for TP which works in Mac OS - tpclient-pyws (yeah I know it could use a nicer name). It's written in Python and uses wxWindows GUI toolkit. I'm also working on another client which I call Parsek and is written in C++ and using Qt GUI toolkit and KDE 4 libraries and so it will also be able to run on Mac OS X.
Our games (or rulesets as we call them) are still quite simple and our clients are not yet as sexy as commercial 4X games, but we are working very hard to make new more complex games and better looking clients. Of course if anyone has skills in graphics, music, programming and is interested in joining our project we would be more than glad to hear from you. Testers and people with new ideas are also always welcome. You can contact us through mailing lists or talk to us on IRC (server freenode.net, channel #tp).