I find it pretty hard to recommend any actual games this year, but odds are that anything from an Apple store will make your gamer happy-- as long as you include the receipt, since even the most standard of computer games seems to have had major issues this year. For the sake of completion, both Freeverse and Ambrosia have a pretty good selection of OS X games to choose from, but I can't say that I really had a must-play OS X experience this year. Maybe next year.
But even if your gamer doesn't play that many games on their Mac, there are definitely gifts to get them.
And the first of those is an iTunes gift card (Apple, $15-$100). Most gamers also tend to be media consumers, so while your Aunt may flinch at receiving a gift card for the holidays (she'll take it as a sign that you don't care enough to choose something for her), most gamers are happy to get gift cards-- their tastes change so fast that you aren't expected to keep up with their media consumption. And a gift card for iTunes also gives them the chance to play Phase (iTunes, $4.99), a music game by Harmonix (the makers of Rock Band and the original Guitar Hero games), and probably the best game to hit the iPod yet.
Mike Rose mentioned both World of Warcraft and EVE Online in his guide for kids, but while those are both great games (and both great experiences on the Mac), you've got to remember that anyone you buy either game for has to pay a monthly fee to play them. So even if your gamer is already playing one of those, you can help them pay their subscription fee with gametime cards for WoW and EVE (Blizzard/CCP, $30-$50). That way, you can give them the most important thing for players of either of those games: playtime.
Finally, the truth at this point is that most Mac gamers don't play games on their Mac-- they play them on their consoles. The hottest selling console this holiday season is the Nintendo Wii (but good luck finding it at retail-- most stores are sold out), and Microsoft's Xbox 360 (I know, it's Microsoft, so it's still buggy, but it is popular) is close behind. And if you know someone who has one of those consoles-- or is getting them-- and a Mac, there are two terrific programs to let them do more with both their Mac and the console.
Wii Transfer (Riverfold Software, $19) has been mentioned before here on TUAW, and is a great way to get your Mac media browseable on your Wii, as well as back up saved games on your Mac. And Connect360 (Nullriver Software, $20) will do the same for your Xbox 360, painlessly and quickly putting any or all of your iTunes playlists and iPhoto pictures right there streamable on your 360. The other console in the running this year, the PlayStation 3, doesn't have a dedicated program to connect up to the Mac, but we've heard that EyeConnect's UPnP compatibility will do it for you just fine.
Unfortunately, we're not to a point yet where there's a must-have gaming experience only on the Mac (although here's hoping that this time next year we'll be recommending some terrific third party iPhone/iPod touch games). But there are definitely options out there for the gamer in your life who happens to appreciate the Mac experience, too.
Update: Mike Rose, smart man that he is, reminds me that the PC versions of Bioshock (Irrational Games, $50) and The Orange Box (Valve, $40) (Portal, as I've mentioned before, is my pic for Game of the Year) should both run pretty well on BootCamp, assuming your gamer has something better than the MacBook's integrated video card. I haven't heard anything about how Crysis (Crytek, $50) runs (and its system requirements are pretty high), but that's the other monster PC game this year.