Apparently Steam is having a "holy crap" sale again where there's just a massive amount of games on sale. The best part is that they always do this when I have no money to spend on games. They should have waited 2 weeks when I'll have lots of money to spare.
Anyways, just hit the front page and you'll notice them all over.
Killing Floor - $5 (Fun, Highly recommended, I don't own it)
Bioshock 2 - $15 (I own the first one(thanks Kyle), it's good)
Trine - $4 (I own this, it's fun)
Overlord(complete) - $4.50 (I own the original, I haven't played it all the way through, but it's pretty amusing)
Most notably though, something that is a MUST GRAB if you have the money right now, is the Valve Complete Package, this includes Left 4 Dead, Left 4 Dead 2, All of the Half Life games, Team Fortress 2, Portal... Everything!
If you look in this list also, It's 75% off the complete 2K Complete Pack, knocking it down to $90, however I don't know most of those games and probably wouldn't play them. However, Borderlands, and the DLC, as well as Bioshock 1 and 2 are in that list, and they all seem to be on sale individually. Borderlands was a LOT of fun.
If you want a laugh, check out the Square Enix / Eidos sale too. All of the games in the list are Eidos, which makes me laugh because the Square Enix logo is more noticable to me than the Eidos one.
Anyways, if you didn't know about the sale, and I helped you find it, I hope you have fun and enjoy.