I finished World of Goo (last Saturday) and Fallout 3 (a couple of week ago). I still have GTA4 to go.
But I am already dreaming of new games I wanna play: Braid is one of them. Then there's The Path (although I have some reservation because it is a horror game).
I still have Call of Duty 4, Mass Effect and Stalker to play. These are heavy duty stuff that probably needed more time investment .. but Braid and The Path would be pretty simple and fun.
Let's wait for the Steam Sales.
At the moment, my attention is on GTA4.