Steam, the games client that was, until today, limited to Windows has been launched for Mac and Linux too.
Now remember, it's only running OpenGL games, and seeing as most are made for DirectX it's going to be a long time until you can get your all-important fix of Call of Duty or tear up a few tracks on GRID, on your Mac. (Without Bootcamp, or other VM for all you wise guys..)
Downloads on the full post,