Here are some games I've played through and enjoyed in my steam library not yet mentioned here:
Euro Truck Simulator 2(Demo Available)
Half Life 2(consider also the original HL, HL2 episodes 1 and 2, Lost Coast tech demo available to try)
Mafia 2(Demo available)
Magicka(Think I got this and SOME dlc in a humble bundle, played the crap out of it)
Orcs Must Die!(There is a sequel as well, but still working my way up the difficulty in the first one)
Quantum Conundrum(Demo available. Portal-like. Switch dimensions to heavy, fluffy, reverse g, slow)
Scribblenauts Unlimited(seems like a kids game, but I spent like 24 hours playing through this one)
Serious Sam 3(also suggest original 1st and 2nd Encounters, still playing through Serious Sam 2)
Star Wars Battlefront(my only experience so far is the SP campaign, but it is decent)
Tomb Raider Chronicles(I have them all, but have only played through that one so far, several times)
I only have about 5 dozen games in my steam library, only 3 or 4 dozen fully downloaded, and an number of them I either haven't started yet or I'm not that far in. Hope this list proves useful to you.
Edit:Star Wars Battlefront 2, but I do have the first one not on steam and have played the crap out of SP modes.