With the Steam service from Valve being such a huge hit on PCs, it was a bit of a surprise when Valve announced that it would bring its incredible service to Macs in April
Not all games will work due to the fact that Windows and Mac OS X are completely different operating systems. Valve is ready to port all of its greatest hits to Steam on Mac including both Left 4 Dead games, The Half-Life Series, Counter-Strike, Team Fortress 2, and Portal which will all be there at launch.
Valve also announced that if you already have bought a game on PC, you can get the Mac version for free and vice versa.
The first game to simultaneously release on PC and Mac via Steam will be Portal 2 which will release this Fall. Portal 2 will also arrive on Xbox 360 as well.