I'll tell you. Nazis, that's who.
Headup Games have created a bridge-building game, Bridge Constructor, and they want to appear on Steam Greenlight. They're still missing a few votes to make it, so they've decided to make the game available for free !
Head over here to download it, all they ask in exchange is that you help them by voting for the game to be sold on Steam (voting is done on that same link, you'll need to login first).
The game itself is not breathtaking graphically, but fun nonetheless.
Not to be confused with another game I talked about recently: