Destiny 2 for PC is free until November 18th through the Battle.net Launcher.

PlayerUnknown's Battlegrounds and Crackdown are free on Xbox right now.

https://www.microsoft.com/en-us/p/play-playerunknowns-battlegrounds-full-product-release-free-for-a-limited-time/c4ff5wjxxq2m

https://marketplace.xbox.com/en-US/Product/Crackdown/66acd000-77fe-1000-9115-d8024d5307dc

Sins of a Solar Empire: Rebellion free on Humble Store: https://www.humblebundle.com/store/sins-of-a-solar-empire-rebellion

search free games on the console platform

Company of Heroes 2 is free on Steam this weekend: https://store.steampowered.com/sale/coh2_giveaway_weekend/

I Am Not a Monster free on Steam for a limited time: https://store.steampowered.com/app/826600/I_am_not_a_Monster/

https://www.humblebundle.com/store/lego-the-hobbit

Free if you're a newsletter subscriber of Humble

Full Throttle Remastered is free on GOG for 48 hours.

Subnautica is free on the Epic Games Store for 2 weeks: https://www.epicgames.com/store/en-US/product/subnautica/home

Their current announced plan will be to have a new game every 2 weeks for free. Next one will be Super Meat Boy.

Anno 1602 Standard Edition is free via the uplay client until Dec. 23rd

Sins of a Solar Empire: Rebellion is free on Steam through Wednesday. I believe it was previously free through Humble: https://store.steampowered.com/app/204880/Sins_of_a_Solar_Empire_Rebellion/

Humble apparently decided we couldn't have Lego The Hobbit without Lego Lord of the Rings, so they're giving that away too: https://www.humblebundle.com/store/lego-the-lord-of-the-rings

Super Meat Boy free is now available for free on the epic games store as announced earlier.

SOMA is also free over at gog.com

And now Super Meat Boy's 2 weeks are up and What Remains of Edith Finch is free on the Epic Games Store until the 24th, when it will be replaced by the Jackbox Party Pack.

Braveland free on Steam until 10 AM PST tomorrow. https://store.steampowered.com/app/285800/Braveland/

Edit: whoops sorry, think I posted in the wrong section. Sorry, I'm new here. Could someone delete this reply somehow?

Distraint: Deluxe Edition free from GOG's main page for the next couple of days.

Epic Games Store has the first Jackbox Party Pack until 02/07, at which point they'll switch to Axiom Verge.

The Fan is free on Indiegala. Note that this isn't a Steam key and is actually a DRM-free download. This game appears to be missing from the database here.
https://freebies.indiegala.com/the-fan

Deponia: The Complete Journey is free on Humble Bundle. As usual, this is a Steam key and will end either when keys are no longer available or on January 26th at 10 AM PST.]
https://www.humblebundle.com/store/deponia-the-complete-journey

Endless Space Collection is free via Games2Gether. Registering on the website and linking your Steam account will net you Endless Space Collection and several DLCs for Endless Space 2 for free. This collection is missing from the database, but for reference, it includes both Endless Space and the Endless Space Disharmony DLC.
https://www.games2gether.com/ (EDIT: Issues resolved)

Kholat free on Steam for a limited time. https://store.steampowered.com/app/343710/Kholat/

Assassin's Creed Chronicles China is free on Uplay right now https://store.ubi.com/us/assassins-creed-chronicles--china/575ffd8fa3be1633568b4cf0.html

Epic Game Store is now giving Axiom Verge away. Free game switches to Thimbleweed Park on the 21st.

If you haven't played Axiom Verge yet, I highly recommend it. Fantastic metroid-vania!

Epic Game Store has changed its free game again. It's now Thimbleweed Park with Slime Rancher coming March 7th.

Slime Rancher is now free on Epic Game Store until the 21st, when the free game changes to Oxenfree.