Cthulhu Saves the World + Breath of Death VII - $3 on Steam.
Bindings of Isaac - $5
BoI Wrath of the Lamb (the expansion pack) - $3
E.Y.E Divine Cybermancy - $10
Trine Complete Collection - $8.74
Evil Genius - $10
Orcs Must Die - $10
Psychonauts - $10 (although, you'd better get it either from GOG or as part of the Humble Indie Bundle 5)
EDIT: Also, the Crysis series is on promotion - each game is less than $10
EDIT 2: Knights of the Old Republic 2 - $10 - it's worth the money. EDIT 3 - also, same price for KotOR 1. I guess, if you really like BioWare, it's OK.
And after actually checking which games were nder ten bucks, here are some more recommendations
Heroes of Might and Magic 5: Tribes of the East - $10 - it's a standalone expansion but it's the best of all. Give you the ultimate Heroes 5 experience
Dark Sector - $10 - meh, if you're into shooters with some supernatural powers, it's alright.
Rune Classic - $10 - it's Rune but...on Steam. Third person fighter with vikings and the ability to beat an enemy with their arms or legs. It's fun.
Deus Ex - $10 - well, old school very classic RPG.
Universe Sandbox - $10 - if you like to play God on a universal level, I've heard it's good.
Avernum 4, 5, 6 - each is $5
Avernum: Escape From the Pit - $10
Machinarium - $10 - if you're into puzzle games, it's quite good. Or so I'm told.
Hitman: Blood Money - $10 - although there are other Hitman games on Steam, this one is my favourite.
But you can safely cross out all the other suggestions, because I just realised Magicka is $10, and it's a totally completely awesome game.