Portal (on a Mac)
I found out that you can play Portal on a Mac without installing Windows using CrossOver Mac. Yay.
(2010 update: Of course, these days we can run Portal (and Steam, TF2, ...) on Mac. And the visual glitches mentioned have all been fixed in newer versions of Portal and CrossOver, too.)
It works fine with minor glitches, except that on the MacBook I have access to, level 19 (the last main-sequence level) appears nearly completely obscured by blue fog. I have not found any solution for this yet, though I've seen a report of success. (The launch option -dxlevel 70 works for Half-Life 2 (which uses the same engine), but is incompatible with Portal.) Level 19 works fine on an iMac.
The worst of the minor glitches was that the first time, it started up in full-screen with the image shifted down and right by half the screen size.
Changing the display options to some other resolution will straighten it out, but the glitch put the Options menu item off-screen. I found that the menu could be accessed by keyboard, and guessed correctly that Options would be the second-from-last option. So, highlight the first option, up arrow twice, return. Then drag the options subwindow by its edge to get to the controls.
Oh, and the game?
Portal is great.
I especially liked the moments of stare at layout baffled ... OH! ... portal-portal-portal-ding!
A bit of advice: Don't go on YouTube and watch a bunch of videos. There are only 19 levels (25 if you count the advanced maps, which are modifications of 6 of the regular ones), and having several of them spoiled was a bit disappointing.
I haven't tried any third-party maps. (Got recommendations?)
Oh, and one tip: when bouncing through a pair of floor portals and trying to aim at something, be looking straight up or down when you pass through the portal - that way, the game won't reorient you while you're trying to aim.