From what i see from the "Games $10 and under" tab on the front page.
Killing Floor - Really fun, especially since the Xmas update, very awesome.
Flashpoint - fuck that game, don't buy any of these games.
Fallout: New Vegas - Really great, one of my favorite games right now for single player.
Amnesia - scary as fuck, play it in the dark.
Star Wars KotOR - i liked it back in the day, but i'm pretty sure that for Windows 7, the visuals are all fucked up, so if they fixed it, get it, but if not don't waste your money.
Orange Box - Do i have to explain?
Mass Effect - better on consoles to be honest, great game, just like it better with a controller :P
Left 4 Dead 2 - like killing floor but better polished and faster.
Battlefield Bad Company 2 - still a great game.
Wow there are way too many games to tell you about, if i were you i would just go with your gut, add me on steam if you want an explanation of a game since i got a few.