I probably have a list but the one that would get me the most slack is that on my Steam account but not on my hard drive is Deus Ex, Half Life 1 and 2, Portal, Braid, and Bastion among other things. Basically, any spiffy PC game released after 1997 is most likely something I haven't played.

I have not played a GTA game extensively since GTA3 even though I actually own Vice City.