In celebration of Steam for Mac's launch, Valve has slashed prices on various titles compatible with both Windows and OS X. We've already mentioned a few of these deals in a post earlier this week, but to recap: Portal is free (no strings attached) until May 24, Torchlight is $9.99 (50% off), Steam Play Indie Pack is $19.99 (71% off), the Telltale Steam Play Pack is $29.99 (57% off), and Quantz is $3 (70% off).
Additionally, the digital distribution house has two PC-only titles, Tropico 3 and Shattered Horizon, marked down by 75% and 25%. The former runs $7.50 through the weekend and is a construction and management sim in which players stand as dictator of an island in the banana republic during the cold war. The latter, priced at $14.99, is a sci-fi multiplayer FPS with players engaging in space-based combat.