I really recommend Warframe. I played it since closed beta for about two years and finally stopped...I've never played a game that long before! Spaceninjas in essentially power armor are awesome :D
It's a third person shooter, and the devs are REALLY cool. They do constant devstreams and are very connected to the community. It's hard to go wrong there, unless you just don't enjoy the genre. Hope you enjoy it!
Spiral Knights, Nosgoth, Hawken and War of The Roses (Kinda) come to mind I guess when it comes to free games actually on steam, that aren't just generic korean MMOs.
Spelunky - indiana jones-esque side-scroller roguelike
Cataclysm: dark days ahead - apocalyptic survival roguelike
Dwarf Fortress - fortress management and adventure roguelike
Planetside 2 - futuristic MMO FPS
Mechwarrior online - stompy mechs
Hawken - spazzy mechs
Blacklight: retribution - beautiful near-futuristic FPS arena with built-in wallhax