Satellite Reign, a spiritual successor to the old Syndicate games, was just patched with a free co-op mode for up to four players. This happened even though they didn't reach the Kickstarter stretch goal for co-op. All in all, it's one of the more successful crowdfunded projects.
It's currently 50% off at Steam.
To anyone that played closer to launch and had performance issues, Satellite Reign was upgraded to Unity 5 a few months back and it now runs a lot better.