Gog.com has another free one for you, Immortal Redneck-with that name you know it has to be a classic.

It's a FPS set in Ancient Egypt of all places. You are a redneck tourist who wakes up mummified in a pyramid. Doesn't that want to make you play immediately?
Also over on Steam, Conan Exiles is free to play this weekend, and Titanfall 2 is under $5 after a large discount.