Serious Sam: The First Encounter free on GoG for 48 hours · 6:58pm Aug 24th, 2020
Never played this one, but man, I've heard people gush for years and years about this series!
(Terribly sorry about the fuzzy stamp quality of the trailer. Was what I could find on short notice before bed.)
So highly recommended if you like your FPSs on the silly, hoard side a la Painkiller or the earlier Dooms.