Local co-op games for 2 players

Broforce, for sure.

Local Co-Op Steam games

Battleblock Theater! :D

Gauntlet is awesome, get a Gamecube emulator and play Gauntlet Dark Legacy. Basically A LOT of the Gamecube games were made for multiplayer so get an emulator. But if its for steam I would suggest Castle Crashers, Lego games like the Marvel Super heroes one.

Adventures of Shuggy
you even can try a free demos on steam

Brothers- A Tale of two Sons.

If you like Fully Destroyable Terrains and Crab-bombs....then Cortex Command is ur game m8

Try Risk of Rain.

Broforce!

Worms, broforce :D
Edit: maybe mortal kombat and/or injustice. If played with girlfriend - Brothers - a tale of two sons may be awesome. It's beuatiful, but sad.

Resident Evil 5 and 6

Hammerwatch is a nice top down shooter with fantasy/rpg theme and pixel graphics.
Nidhogg is a very fast fun sword-fighting game (you propably heard of it).
I can recommend both.

funny no one has mentioned borderlands