Judge Dredd: Dredd vs. Death
It's the *only* PC game developed by Rebellion I hadn't ever even heard about before I discovered it on GOG and also none of my gamer friends knew it, neither when it was new nor when I asked around after discovering it here so I presume that at least the PC version is a rather obscure game (it seems like the console versions, particularly the Xbox version, were a little more popular). And yup, the game is awesome. It's simple, it's stupid and yet it has its clever moments and most importantly it's simply FUN.
Also: Darkstone
Hadn't ever heard about it until its GOG release and I was surprised to see that it's a *really* good Diablo clone. It may not have been groundbreaking (despite supposedly introducing 3D to the genre) but it's very enjoyable, has a few very original ideas and the graphics, aside from the tiny screen resolution, hold up remarkably well. Also it has a fantastic atmosphere. If only the plot and the enemy designs were a little better.
JudasIscariot: Knock Knock is pretty cool too but it's definitely one of those games best played at night with headphones on. Even though it's done in an animated style, it still managed to send shivers down my spine a couple of times :)
You siss... aaaaargh! Yeah, actually I have to agree on that one. The game genuinely scared me a few times. It may not be quite as disturbing as some "real" horror games but the fact alone that I have NO idea what's going on creeps me out. Seriously, two hours in and I'm still not sure what I have to do.