Free steam games.

Check back on February 3rd, when the monthly unlocks the rest, for more possible games.

Else Heart.Break()
Rust
Stronghold Crusader 2*

*On the fence about this one, it looks interesting but also extremely confusing.

Steam is required.

Stronghold series is a good one but all the reviews for that game are on and off. It isn't a confusing game i was actually able to get a pretty solid grasp of it at the age of about 7 or 9.