Clive Barker's Undying added to GOG as the bane of your pants
Clive Barker's Undying carries the distinction as one of the first games to carve an expression of terrified horror in my face from start to end. I swore I'd never revisit it, but it's on GOG now for $6, an absolute steal for one of the best and truly frightful first-person survival horror games out there.
Sure, it's not as weird as previous new(old?)comer Omikron, but Undying still keeps its magic of piling on creepy mysticism, nightmarish enemies, and lots of frantic shooting. All because the Covenant siblings couldn't resist blurting out a profane ritual. Silly Covenants.
As with the rest of GOG's library, Undying's olden 2001 graphics are kept pristine while taking a bump in compatibility for Windows XP, 7, and 8. For more nostalgic incoherent screaming, here's the intro trailer.