Brutal Doom is a famous gameplay mod that adds extreme gore and agressive mechanics. Project Brutality (ModDB) is an addon to that that brings in more mechanics. Hidden here because the creator of the former is an asshole.
From the same creator, Brutal Bolognese is Brutal Doom's gore as a standalone gore mod, and is the successor to the original Bolognese and Ketchup.
UAC Labs (Direct download link) (From here). Hidden here because the creator did the Columbine high school massacre.
NSFW mods: HDoom, Imp Encounter: Second Edition