Best free games 2019?

"Sven Co-op" on Steam. Not exactly from 2019, but it got some major updates this year, so itkinda fits.
https://store.steampowered.com/app/225840/Sven_Coop/

It's a free Half-Life's coop with a lot of mods, difficulty settings, campaings from other mods and so on.

Quite fun. And very addictive - it's easy to not notice how you skipped over 10 hours, playing this thing.