I loved Half-Life 2. I loved Counter-Strike 1.5 (and 1.6 too). I loved Portal. Valve has done a good job of making games. But I reeeeally wish some other developers would step up to the plate and port as well. I'm thinking Borderlands, Deus Ex: Human Revolution, and others. Those two already have Mac ports so I doubt it's unfeasible to move things over. Wine typically works well enough --assuming you're willing to either fiddle with it for hours or else pay for an out-of-the-box solution like Crossover-- but nothing beats native :)

Edit: Been playing the Portal Beta. The horizontal tearing without vertical sync is annoying. Mainly because, with vertical sync, there is substantial input lag. But hey, it's called "beta" for a reason.