Yes, we know that's 2, and not the original, but apparently it's taking Gabe Newell's trained monkeys a little longer than expected to port that one over and get it all compatible with OS X. But, y'know, it's coming, so that's a good thing - and now you've got the sequel to keep yourself busy!
On the other hand, if you haven't yet ventured into the world of the survivors and the infected, Valve have a treat for you. Simply purchase a copy of the game through Steam, and you'll get the game's original five campaigns plus the three DLC levels for your gaming pleasure.
And there we have it. The Steam Play banner flutters out across a land of Mac gaming that grows richer by the day. All of Valve's Source titles have made their way over from the PC, so what are you most waiting for now?