GOG and the Humble Store each have their own separate, unrelated promotions going on where you can pick up a free PC game for free.
From Humble, you can get XCOM: UFO Defense. Also known in some regions as UFO: Enemy Unknown (not to be confused with the 2012 Firaxis game, XCOM: Enemy Unknown), this is the original game in the XCOM series from developer MicroProse Software.
You can claim a free copy here from now until 10 AM PT on Wednesday, February 1. You’ll receive a code to redeem on Steam.
Over on GOG, the free game is Constructor. It’s a touch newer than XCOM, having in released in 1997 (versus 1994). It’s a city-building game with a bit of a twist, as you’re competing with other characters that you can impede through sabotage and other means. A remake is on the way for PC, Xbox One, and PS4.
You can claim a free copy of the original Constructor here until 5 AM PT on February 1.