The reason no games are designed for mac's is since Mac's are still less then 10% of the computer population. Even if every mac user bought a game the publisher would sell more copies buy letting it be on Windows. Hench why you rarely ever see real computer games on mac's.
There are missing some very important ones as far as I see.
- Warsow ( warsow.net ) a fast, quake like fps
- Battle for Wesnoth ( http://wesnoth.org (seems down atm, wiki)), fantasy turn-based strategy
Both very well done.
Oh and C&C Red Alert (1) is for free now, but its quite old.
/e oh and they run on OS X since they are open source. Warsow still beta on mac, but it should work without problems by now.
I don't remember where exactly I saw the link, but there is a copy of SimCity 2000 for Mac downloadable from the EA web site. It requires Classic or a pre-OS X version of Mac OS, but still is fun even a decade and a half after it was first released.