So far Steam has announced a few games that will be coming to Mac users when Steam comes out of beta and is officially launched for Mac users. Although there is a possibility that the Steam Store may sell Mac-exclusive games in the future below is the list games that have been confirmed for Mac OS X:
+ Counter-Strike (presumably Source)
+ “The Half-Life Series”
+ Left 4 Dead
+ Left 4 Dead 2
+ Portal
+ Portal 2 (upcoming)
+ Team Fortress 2
+ Madballs in…Babo:Invasion