Talk of the PS4 and Xbox 720 have surfaced this weekend, so you're in on the coat-tails with current gen console stuff.
I loved these:
Killzone 2 and 3 (actually scary) - got 2 to play
Resistance: Fall of Man - Might get 3
Metal Gear Solid 4 (obvs) - In the 'to play pile'
GTA 4 and all the extras - Not a fan of the series
Dead Space/DS2 - got both for PC, too scared to play them
Heavy Rain (not as clever as everyone makes out but still fun) - mebbe
Batman Arkham Asylum/Arkham City - completed both on PC
Deus Ex - completed on PC
Mass Effect 2 and 3 - completed on PC
Dargonage Origins/Dragonage 2 - got 1 and the addon pack but far too time consuming to play properly
New ones you might dig include XCOM - Enemy Unknown, Journey, will post if I think of any more.
Yeah, I got mine cheap because of the new ones on the horizon. As well as the stuff above, I got Bayonetta and Final Fantasy Xiii-2 and may well order Ico/Shadow of the Colossus too. Antisocial single player games ftw.