Crackdown 2 is free on the Xbox Store. I don't know for how long and whether or not you need Xbox Live Gold: https://marketplace.xbox.com/en-US/Product/Crackdown-2/66acd000-77fe-1000-9115-d8024d5308bc?noSplash=1

It is NOT Xbox One compatible, but given that the first game is, it probably will be eventually.

EDIT: It appears I was wrong and the list I was going by wasn't updated, as it is showing up as a download on my Xbox One.

GRID 2 free on Humble Bundle. It also comes with 2 DLC packs, so even if you have it, it might be worth using the key: https://www.humblebundle.com/store/grid2-spa-bathurst

Epic Games Store now has Oxenfree for regular old free until April 4th. It will then switch over to The Witness for 2 weeks.

The Elder Scrolls: Morrowind is free through March 31st! I lost my disc so I'm pumped that this came up free.

https://twitter.com/ElderScrolls/status/1110303201486663681

The Witness (2016) is now free on Epic Game Store. Transistor replaces it in 2 weeks.

Ubisoft is giving away uPlay copies of AC Unity after the Notre Dame fire. https://news.ubisoft.com/en-us/article/348227/supporting-notre-dame-de-paris

Epic Game Store has switched its free game again. Transistor is now free for 2 weeks until May 2. World of Goo replaces it then.

Kathy Rain is currently free on steam for a limited time

https://store.steampowered.com/app/370910/Kathy_Rain/

Age of Wonders 3 (steam key) is free to anyone who subscribes to the Humble newsletter for a limited time.

Ubisoft is giving away Steep on PC (Uplay) free for the next few days: https://register.ubisoft.com/steep-giveaway/en-US

Last day of World of Goo being free. Then from now until June 13th Epic will be giving a free game every week instead of 2.

It looks like Stories Untold is the next Epic Games Store freebie (until May 30th)

GOG - The Witcher Enhanced Edition
https://www.gog.com/gwent-welcome-bonus?pp=2c61422714bcb5935215adf442a8f5b1d80ffc54

Guacamelee! Super Turbo Championship Edition is free over at Humble Bundle if you're subscribed to their newsletter.

Ball laB is free on Steam through tomorrow: https://store.steampowered.com/app/780800/Ball_laB/

The Sims 4 (Standard Edition) is free on Origin for the next week: https://www.origin.com/usa/en-us/store/the-sims/the-sims-4

GRID 2 is currently free on Steam. DLCs are on sale: https://store.steampowered.com/app/44350/GRID_2/

Rime is now free on the Epic Games Store

Jalopy free for Steam on Humble for the next 2 days: https://www.humblebundle.com/store/jalopy-free

EDIT: Apparently, just a free DRM download, not a Steam key.

Subscribe to newsletter to get Robonauts free on eShop, get 100% off Geki Yaba Runner Anniversary Edition when it releases on the 31st. Both games for Switch.
https://qubicgames.com/

Obduction is currently free over at gog.com along with the start of their summer sale festival

City of Brass is currently free on the Epic Games Store

New Twitch Prime Games for the month of Jun 3 - Jul 1
Stikbold
Aegis Defenders
10 Second Ninja X
Metronomicon

There's actually a separate thread for the monthly Twitch Prime games, but might as well do them here I guess. Who doesn't have Amazon Prime these days?

And on the topic of Twitch Prime, if you grabbed the 3 free months of Nintendo Switch Online when it was first announced, you should be now able to grab the 9 additional free months making a free year.

This WILL stack with an existing Individual Subscription (currently, I'll be next billed in November of 2020), but not a Family Subscription.

Note to Moho: All the above Twitch Prime games are already in the database.

Polygoneer is free on Steam the next two weeks: https://store.steampowered.com/app/684680/Polygoneer/