Killing Floor. I bought it retail, so I suppose I didn't exactly buy it through Steam, but the fact that you need Steam to run it makes it applicable in this case I guess. As for why, well, I had it for about six months before loading it up for the first time around Christmas last year, and since then have gone on to rack up over 800 hours in it. My game time can be divided up into Killing Floor and games that aren't Killing Floor.