10. Helldivers

Enjoy squashing some bugs with your friends.

From the creators of Magicka, Helldivers is a cooperative top down shooter where you must work to save Super Earth from its enemies in an intergalactic war. Up to four people can play together locally. There's an online co-op mode as well. But beware, enemy fire isn't the only thing that can hurt. An unlucky shot from a friend can take you down as well. The levels are procedurally generated and missions are nonlinear, creating a large universe that you get to explore at your own pace. 

Try not to get squished yourself.