To celebrate the release of Mac Steam, Portal is free until May 24th, for both PC and Mac.
[Update: also, Torchlight is half-price for the weekend, and although it doesn’t sync saved games between Mac and Windows, it at least runs well on both. User-made mods have some issues on the Mac side, including differences in how the engine compresses data, and how it handles mixed-case filenames. Still fun, though.]