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.
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.
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.
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.
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.