Well.. tbh I must say I was a gamer, ofcourse some free time still is needed, but... I neither have any good games for the mac, I mostly game on my windows PC, or just the iPhone when im not home ill have a look what games I have on the mac tomorrow - I'm studying atm ( Dont ask me how I got here meanwhile LOL )
Try assault cube it's an fps and it's a bit like cod but there is only 5 weapons and their is no campaign try. I only play minecraft and assault cube.
It seems Ive deleted a few games, anyway. Here is what I still have on my MBP 13" http://www.pictourl.com/clean/08646fbe
Look up armament 2 day z mod it is not free but is the best game iv ever played. Look up youtube video on how to get it if you do get it
Also try cube 2 it's free and from the same makers of assault cube and best of all it's Mac compatable (sorry about spelling typing this on a iPad)
Slender! Even if you beat the game, you lose! Of course unless you get scared before playing the game.