Valve’s Steam gaming service finally arrived on Apple Mac OS X, allowing Mac game to play titles like “Portal without running Boot Camp” or setting up a Windows partition. Steam’s Mac library is kicking off with Valve’s own Portal — available for free until May 24th! — Sid Meier’s Civilization IV, and Runic Games’ highly-acclaimed Torchlight. More titles will be added each week, including Team Fortress 2 and Left 4 Dead 2. Also, Portal 2 will launch simultaneously on Mac and PC versions of Steam. You can grab the Mac client from steampowered.com and start downloading games right away.