DO NOT CLICK The following mods I am not comfortable with on the main page, but want to mention somehow (even if only for completness)
Brutal Doom is a famous gameplay mod that adds extreme gore and agressive mechanics. Project Brutality is an addon to that that brings in more mechanics. Hidden here because the creator of the former is an asshole.
UAC Labs (Direct download link) (From here). Hidden here because the creator did the Columbine high school massacre.
Items below are honestly more morally sound
NSFW mods: HDoom, Imp Encounter: Second Edition
Regular cursed mods: NFT Doom