Here is something new I found right now. It's called Moonbase Alpha, you can play it free over Steam and it features coop. Have a look yourself how much fun it could be.
Step into the role of an explorer in a futuristic lunar settlement...
Your mission is to restore critical systems after a meteor strike cripples life support equipment.
Download link: http://store.steampowered.com/app/39000/
Homepage: NASA Moonbase Alpha