TheFirstMonk

For Honor is free on Steam until August 27: https://store.steampowered.com/agecheck/app/304390/ FYI, it's the Starter Edition that is free. If you're planning on unlocking everything for free, the Starter Edition requires 8000 Steel (in-game currency) to unlock base characters (aside from Warden, Raider, and Kensei, which are all available from the start) and 15000 Steel for DLC characters. I recently unlocked a few classes when they had a discount, where 8000 dropped to 5000 and 15000 to 10000. They are adding a 4th faction of Chinese-based characters in October; those are also 15000 Steel each.

Xenonauts is currently free on GOG for the next ~36 hours: https://www.gog.com/game/xenonauts

Yet Another Zombie Defense is free on Steam until May 11: https://store.steampowered.com/app/270550/Yet_Another_Zombie_Defense/ This is not the HD version (which is 0.99 on Steam).

I only come back to post on the forums to remind people to pick up free games that they will probably never play. You're welcome.

Satellite Reign is currently free on the Humble Bundle store: https://www.humblebundle.com/store/satellite-reign Mixed reviews on Steam, but I think it has multiplayer, so you can enjoy/ grumble your way through the game together. Up to 4 players at a time.

Free Spec Ops: The Line at Humble Bundle: https://www.humblebundle.com/store/spec-ops-the-line Deal ends in less than 48 hours.

Darkness 2 is free at the Humble Store for less than 48 hours: https://www.humblebundle.com/store/the-darkness-ii?hmb_campaign=thedarkness2_freegame&hmb_medium=banner

Assassin's Creed Black Flag is free on Uplay until December 18: https://store.ubi.com/eu/assassin-s-creed--iv-black-flag-tm-/57062e8188a7e316728b459e.html

Fallout is free on Steam: http://store.steampowered.com/agecheck/app/38400/

Outlast Deluxe Edition is free on Humble Bundle: https://www.humblebundle.com/store/outlast-deluxe The base game and the Whistleblower DLC are included in the Deluxe Edition. EDIT: Credit to Jackie since I did not see his message at the right of my screen until after I posted.

Back to Bed is free on Steam: http://store.steampowered.com/app/308040/Back_to_Bed/

Oddworld: Abe's Oddysee is free on several platforms: Humble Bundle GOG Steam Limited time only.

Psychonauts is free on Humble Bundle: https://www.humblebundle.com/store/psychonauts Deal ends in less than 48 hours.

Mirage: Arcane Warfare is free on Steam: http://store.steampowered.com/app/368420/agecheck Deal is available only on 9/6/17.

Steamworld Dig is free on Origin: https://www.origin.com/usa/en-us/store/steamworld/steamworld-dig/standard-edition