Portal 2 - Brilliant puzzle coop, but once you've finished it it's done so this perhaps doesnt fit your criteria and it's not that cheap atm.
Terraria
Baldurs Gate 2 - I had a lot of fun playing through baldurs gate 2 with friends. It's still the best RPG experience to date imo, the only downside being that one player will play as the main character so will make all the decisions, but I'm sure you can come to an agreement.
Magicka is hilarious in coop.
Alien Swarm is pretty good for a free game.
BC2 isnt exactly coop but it's excellent for going around as a duo, tearing **** up in tanks, boats, milk carts etc. and really rewards tight team play between 2-4 people.