Space Engineers license, according to developer Keen Software House. The game celebrates the sales milestone in conjunction with its one year anniversary of availability on Steam Early Access, where it still resides.
"Looking to the future, Space Engineers will remain in development for a while longer," Keen Software noted today. "Focusing on adding new features and bug fixing and also continue taking feedback and suggestions from the community who is helping to further improve and expand the game."
The PC game was also mentioned as being part of the Xbox One's Indie line-up a couple months ago at E3. We recommend it for the ability to smash spaceships into each other.