Valve's Steam for Mac is available for free, and they're giving Portal away for free for the next two weeks. For those who don't know, Steam is a kind of iTunes for games. While their entire catalog isn't available for Mac yet, they are working on it. Released as part of the Orange Box a couple years ago (along with Half Life 2 and a couple of expansions, as well as Team Fortress 2), Portal is a first-person-shooter-style puzzle game.
I must say that it feels a bit clunky, and the scrolling is very twitchy and weird, but a fair first release. (Their servers are taking a beating, but I won't ding them for that... Especially on release day.) It also eschews some of the Mac UI conventions, and is guilty of not feeling like a native Mac app. But with that said, it is at least attractive. I'm willing to bet that it will get better in very short order.
If you have an Intel Mac, you owe it to yourself to pick these up. (Sorry, but no Linux love...)