Steam Platformer / Adventure Games

Super Crate Box is a great new F2P that is a platformer I believe.

Trine

Super Meat Boy? Also, Braid

Just forget Steam and download Treasure Adventure Game. You can find it either on Tiggit or if you google it then as a free download on a lot of sites. It's free, but I wouldn't have minded paying for it actually. A very good game to say goodbye to the summer with.

Cave Story+

Cave Story +
Braid
Oddworlds
Capsized
NyxQuest: Kindred Spirits
This genre is my favourite ^^

VVVVVV

Psychonauts.

not on steam, but if you didnt play them, kirby 64, yoshi's story, and paper mario (all except the wii version)

Super Meat Boy

Limbo!

Rayman Origins.

Most of the aforementioned, as they're solid suggestions; on my part, Tobe's Vertical Adventure was fun enough to see to it's end (it's short - though I suppose the time trial achievements might give it some more replay value), Sideway is an interesting take on the genre, Snapshot sounds like fun (but I've yet to try that one so not giving a full-out recommendation), VVVVVV if you don't mind the old-school graphics and... technically not a platformer but Aquaria might be of interest to you (personally, I loved every moment of it)