Ones that I can speak for, Trine is pretty good at $8. Puzzle platformer. Kind of more platformy and less puzzle version of Three Vikings.
Torchlight is Diablo 1 redone as a pick up and play lunch game that you can play all night, it is very good if you are into that kind of thing. $10
Indiana Jones and the Fate of Atlantis is $2.50. It is Lucas ARts point and click adventure and Indiana Jones, take that as you will. Actually forget that
http://store.steampowered.com/sub/2102/ Get this instead, it has that in it and you get 3 other games for the same price. HOly fuck that is a good pack for $2.50. 2 Indiana Jones games, Loom and the Dig.
Holy shit STALKER is $1.50. It isn't amazing, but that is a pretty hard to beat price. Shooter that takes place in a restricted area around CHernobyl, that one is probably worth the price of entry.
Beyond GOod and Evil is $5. IF you haven't played that GET THAT. It is a good game.
Secret of Monkey Island is $5. If you don't have it and liked Monkey Island 1 or have never played Monkey Island then you should get that.
Assassin's Creed 1 is $10, if you want something with a lot of shiny to it then this is a pretty good game for it. It is shallow, but it is pretty fun.
Chronicles of Riddick Assault on Dark Athena is $6.70 and I actually recommend this since you get the original Riddick game done in the same engine for free with it and that itself is a pretty good game. I am still getting to Dark Athena (working my way through Butcher Bay occasionally). Good value.
Serious Sam HD at $10 is a good buy if you like your old school style shooters.
Star Wars Jedi Knight collection at $10 is 4 games and one stand alone expansion pack and they are all pretty good shooters. Very good value.
Edit - Freedom Force is $3.50 and is a neat little comic book squad based real time game.
Witcher is not bad at $13.60. Hack and slashy RPG where you play the Medieval equivalent to James Bond.
Stuff I can't vouch for though.
Eufloria is an indie little art fag type game that I have heard good things about, $10.
AaaaaAAaaaAAAaaAAAAaAAAAA!!! A Reckless Disregard for Gravity is $7.50 or in a pack with something else for $11.30, it is... something.
World of Goo you probably have caught a bit of cursory drama about Piracy and stuff, it is $5. Apparently very good.
The Path is $10, but doesn't seem to be on sale.
X2 the Threat is a space shooter type thing, it is apparently not bad. $2.50