Hmm, well I guess it depends on how "non-D&D" you mean. The first thing which springs to mind is the Book of Ebon Bindings for Empire of the Petal Throne. It is a chillingly realistic-sounding tome on Tsolyáni demonology, right down to all the gruesome details. Demon after demon is detailed, complete with weird and grotesque summoning rituals.
Or, there are the excellent paper miniatures at iheartprintandplay. I love the Order of the Stick style art for the figures. They are a refreshing diversion from the usually serious and realistic art you get for RPGs. I think of these as the perfect miniatures for playing Old School Hack.
Or, lastly, the Abyssals supplement for Exalted. The Abyssal Exalted are souls of persons who took a dark bargain at the moment of death and now are Deathknights serving the underworld. Although they are bound to their masters they still have free will and drives and desires left over from their tragic previous life. If I ran an Exalted campaign it would be an Abyssal campaign.