Seeing as a couple of multiplayers are released on Mac now, you can play with your friends who prefer half eaten apples to windows.
Most noteworthy multiplayer: Killing Floor (which is 33% off). All of the releases are on sale so get them now if you haven't already!
And yes, I like alliterations.

On an unrelated sidenote: Anyone got a clue about the engi update for TF2? For god's sake Valve are being slow again.