If you are a fan of The Dark Mod, a Thief-inspired Doom 3 mod that is now standalone, you need to check out this list of scary mods to celebrate Halloween. Mod DB put together a list of its favorite horror-themed mods for The Dark Mod, and they are all worth your time.
Spend tonight creeping around haunted mansions, exploring crypts, fighting skeletons, and being watched by ghosts. It's completely free to download The Dark Mod and all of these mods, so you should really check them out. I mean, free horror games on Halloween? How can you go wrong?