Actually, probably an online text-based MUD. I can't remember which of them was the really dark world, but that doesn't really matter, because a huge amount of the content is user created, so everyone invents these really twisted histories and storylines for themselves. One of the characters I knew in the game had apparently been trapped in a tower by an evil villain for decades and held captive as a sex slave, leaving her traumatised and half-insane. oO It's twistedly awesome. =D
As far as pre-written games, it's hard for me to judge. The darker a game is, the more I tend to enjoy it, so I don't even usually notice, but, well, for games that are ostensibly kid-friendly, MediEvil and the Oddworld games are both loaded with dark humour, which is probably why I love them so much. =3
Seriously, the fact that they stitched the eyes and lips of mudokons shut, and intended to market them as a food product probably qualifies them as being pretty dark. Plus, the morbid humour still makes me laugh my ass off. "Our spew is your brew!"