Board Games

I'll echo some suggestions and add some more. Before that, there is something to note about many coops; you can divide them into two groups, first, could I play by myself (e.g. Pandemic and controlling multiple characters), and second, that they require multiple people because of hidden information (e.g. Hanabi). Almost all of my coop suggestions fall into the later.

Just One - @roarmalf pointed this one out, and I second it. This has gotten an extensive amount of play in our group.
Quirky Circuits - This is a real time game for the action allocation portion where you play your cards. More players is better IMHO. You're playing cards to program the little roomba/robot that is cleaning the house and then you flip them up to see where it actually goes.
Rosetta: The Lost Language - I think it's a marriage of Mysterium (coop) and First Contact (competitive) as a small box coop. Incredibly group dependent, but there is a print and play copy (of the first edition) available for $5 from a different publisher who is going to do a second edition in the future. this one scales well once you get 3 total people as a bottom threshold and players can enter and drop out of the game (other than the story teller) at will which is a nice feature. Some people will like this more than Mysterium (and it has 1/4th of the rules), some will like Mysterium more.
Magic Maze - Kasper Lapp has a new one called Gardeners that I really like, but it's not available widely yet where as his earlier game that is quite similar called Magic Maze is. Players can't communicate except by tapping a wooden peg in front of another player to alert them that they should be the one doing the next move. otherwise it's unstructured as to who should be taking the next turn (oh, and it's real time).