For only a few more hours, the colony city builder Surviving Mars is free on Steam.
Publisher: Paradox Interactive, Developer: Haemimont Games, Abstraction
Overcome everything that the red planet throws at you, to establish a base for humanity as they reach for the stars. There are also several DLC for the game, once you've done that!
It's been given away via Humble Bundle, so you may already own it, but for everyone else you only have until 6pm BST to grab it on Steam.