https://store.steampowered.com/app/1066920/Minion_Masters__Accursed_Army_Pack/
A Minion Masters DLC.
Minion masters itself is permanent free to play

Obduction is free on GOG for another 39h

Polygoneer on Steam until June 19.

Kabounce on Steam for about 48h

OverCooked Free2Play on Epic Game Store.

They release a free game every 2 weeks

EDIT:
https://www.epicgames.com/store/en-US/product/torchlight/home Torchlight F2P!

EDIT 2:
https://www.epicgames.com/store/en-US/product/limbo/home Limbo F2P!

Hot Shot Burn Open Beta for Steam:
https://www.reddit.com/r/FreeGamesOnSteam/comments/cee74h/hot_shot_burn_open_beta_link_in_comments/eu1vtz1/
Simply join the Discord and you'll automatically get a code through a PM; got mine today.

@ Medeiros: An open beta isn't a free game...

Moonlighter AND This War Of Mine are currently free in the Epic store!

@ DC: you can install and play the game on Steam without paying for it. Is that not free?
At least before it goes into Early Access it's completely free, not sure about the pricing afterwards. I gave it a try and it's a really fun game with tons of potential.

Surprised to see this thread not being that active anymore, there were plenty of games that were given away during this time.

Endless Space 1 Collection
Turn-based 4X strategy game in space
Steam key given away on Humble Bundle (Link)
Ends on Sep 14th, 2019, 17:00 UTC.

Conarium
First-person puzzle adventure with a Lovecraftian horror theme
Epic Games Store weekly free game
Ends on Sep 19th, 2019.

The Great Geometric Multiverse Tour
Arcadey singleplayer FPS with roguelite elements and trippy low-poly graphics
Steam Store (Link)
Ends on Sep 16th, 2019, 17:00 UTC.

Well, if you missed the offer for getting DiRT Rally for free from Humble Bundle, now's your 2nd chance... on Steam.

GTA San Andreas for free

Various Batman Arkham (Asylum, City & Knight) and Lego Batman (1-3) games are free on Epic store right now!

@ Medeiros: You can't play a Beta forever in most cases (especially if it's online) and it's meant for testing in first instance. It can contain bugs and is probably incomplete and/or unbalanced. I think a beta is more like a demo but not what a fully free game looks like.

As usual: Epic Games made 2 games for free!

Metro 2033 Redux
and
Everything

If you enable 2 factor auth on Origin this month (October 2019) you'll get a free month of EA Access in November. If you already pay for it you'll still get that month for free.

https://help.ea.com/en/help/account/login-verification-and-origin-access/

One month actually only costs 3,99€ normally (in Germany, and only ~2,08€ if you pay for a whole year) but it allows you to play full versions of A LOT of good games:
https://www.origin.com/store/browse?fq=subscriptionGroup:vault-games

No advertisement for EA. I dislike EA (they destroyed C&C. Never forget!) and I also dislike the Origin client but I was quite surprised how many games you can play for that little money

Minit
https://www.epicgames.com/store/en-US/product/minit/home

Just imagine you would create a "Random Game Key" for 1,99$ which only contain games that were free at some point.

Princess Remedy 2: In a Heap of Trouble.

Free until October 15th

https://store.steampowered.com/app/522040/Princess_Remedy_2_In_A_Heap_of_Trouble/

https://www.epicgames.com/store/en-US/product/surviving-mars/home

https://store.steampowered.com/app/490860/DashBored/

Free until the 4th of November.

https://www.epicgames.com/store/tr/product/soma/home

https://register.ubisoft.com/rabbids-coding/en-US

+ Another game on epic store free 2 play but not Soma! and forgot name and Im to lazy to investigate for the name but rather type random bullshit