Stellar Mess: The Princess Conundrum (Chapter 1) free to keep when you get it before 27 Feb @ 7:00pm (CET).
https://store.steampowered.com/app/1507530/Stellar_Mess_The_Princess_Conundrum_Chapter_1/
You can get free Distant Space 2 action shooter forever on Steam until December 24th at 7pm (CET) :
https://store.steampowered.com/app/729460/Distant_Space_2/
Intravenous stealth tactical assassin game available for free forever on Steam until July 16 at 7pm.
https://store.steampowered.com/app/1486630/Intravenous/
Nomads of Driftland: The Forgotten Passage fantasy RTS scenario pack free on GOG for the next 144 hours:
https://www.gog.com/#giveaway
Base game is also free: https://www.gog.com/en/game/nomads_of_driftland
Unity of Command: Stalingrad Campaign Steam key is free to get on Fanatical for a week:
https://www.fanatical.com/en/game/unity-of-command-stalingrad-campaign
For a limited time, the Steam code for the Hotshot Racing car racing game is now free to collect on Fanatical: https://www.fanatical.com/en/game/hotshot-racing
Battlestar Galactica Deadlock space strategy is free on Steam until April 9 at 7pm (CET).
https://store.steampowered.com/app/544610/Battlestar_Galactica_Deadlock/