Have you taken a look at Steam ?
There are quite a few games for OS X and they seem to be adding more and more.
They have a nice community forum.
gaming.stackexchange.com/users/749/Martin Marconcini
Steam has started supporting and distributing Mac games a couple of months ago.
You can use the Advanced Search to filter by OS.
At the time I am writing there a 138 games and 18 DLC for Mac listed there.
On 19 June 2010, Appstorm.net wrote an article about 50 Games for Mac .
Wikipedia has a long list as well.
Pay attention that not all games run on all Mac models.
Check the requirements before buying them.
gaming.stackexchange.com/users/730/Drake
GiantBomb's games list, filtered for the Mac platform might be a good place to start.
Since it's a wiki, everything stays pretty nicely updated even if it's an old game that's been ported through Steam or some other intermediate platform.
gaming.stackexchange.com/users/1070/authenticgeek
This question was quite a while back
but there's also the Mac App Store now, which has a game category of it's own.
To be fair, there are lots more mini-games (think iPhone game turned desktop) than full-size games, but a couple high-profile games like Call of Duty 4 made it in for the launch and surely there will be more soon.
gaming.stackexchange.com/users/1070/authenticgeek
I'm not sure if there is one "end all" site for all known OS X games, but an additional source to consider is Steam Mac section .
There isn't a huge selection yet, and it's mostly Valve's own games, but the list is growing each week.
One nice thing about Steam is that if you buy a game, you can play it on all supported platforms without buying an additional copy.
gaming.stackexchange.com/users/461/Adeese
If you are looking for a place to buy older PC games that may or may not run on the Mac I would checkout gog.com .
They seem to some how get publishers to port most of their games to OSX Reasons why I love gog.com: You buy it.
You own it.
100% DRM-free.
You get awesome content (soundtracks, wallpapers, manuals, videos, etc)
Extra compatibility (Window XP or later, Max OSX 10.6.8 or later)
Now if they could get ATARI to port Roller Coaster tycoon already.
gaming.stackexchange.com/users/16682/John Riselvato
I'm looking for something like  http://gamerankings.com/  where I can find comprehensive information about current (and past) games, but for Mac OS X. Not necessarily just a site linking to other reviews, but it shouldn't be a time sink like normal gaming sites, either.  http://apple.com/games/  and  http://insidemacgames.com/reviews/  come to mind, but the former seems to be either incomplete or outdated, the latter very puzzle- and casual-heavy.
How can I find out about current Mac OS X games?
gaming.stackexchange.com
<macos>
gaming.stackexchange.com/questions/2708
gaming.stackexchange.com/users/1177/Daniel Beck
You can try Steam, wikipedia and the Mac App store. Steam now supports Mac games, and there is of course the Mac App Store too. However there are some lists out there too, one on Wikipedia and one on GiantBomb's games list.
There are a variety of ways with which to find games on Mac.