Space Hunk from Mojam 2 =P

Liero
Real Time Worms. :o

Magnetis

Obscure 1, 2, Tony Hawk's Underground 2, Worms

Adventures of Shuggy has co-op mode. I sometimes play it with my brother.

Lego games

Lego Star Wars Complete Saga, Worms, hotseat play in Sid Meiers Civilisation games, older racing games sometimes allow 2 players (one on the arrows, one using the number pad or WASD)

For worms, set yourself and other person as one team of teams / alliance etc. and play against the computer or have an agreement to kill the computer first then kill each other at the end.

Golden Axe series allows for two players to play simultaneously while sharing one keyboard. It's available on Steam.

F1 Race Stars, but you need cont. Great game for sure.

For using one keyboard, I recommend turn based games like Worms or Civilization.

Super smash brothers

Use N64 emulator

Heroes of might and magic 3. Probably best game ever made.

Bang Bang Racing

Rayman Origins?

Some non-steam stuff like Megabyte Punch(on desura) or Space Hunk(DRM-Free, hit me up if you want it)

Curve Fever is a cool game. It is like snake but a match is for up 8 people! Have fun with it :)

Teenage Mutant Ninja Turtles(2003). Epic game. But very difficult.

EDITL Also, try GISH versus mode :D Pretty fun

This search works pretty well for steam games: http://store.steampowered.com/search/?term=local&category2=9&snr=1_7_7_151_152
Rayman Origins, awesomenauts, and Trine 2 are good.

Rayman Origins - very good co-op game :)

Mightandmagic clash of Heroes. Appart from the old ones
Trine s
Kung fu strike. I thought the level of the two dark elf bitches was impossible in co-op (each divide in 2 and they are always kicking), but my mate and I achieved it and can continue enjoying it now after the holydays pause.
EDIT: I recently added sacred citadel to my wishlist as it seems will be a good game for the purpose you and I search.

disciples 2

Gundemonium series

Shatter, Jamestown

Wyv and Keep.

Bubble Bobble if you want to go old-school.

You can play Eragon, it is little old but its good game.