Brutal Doom is a mod created by SGtMarkIV which adds new weapons, animations, and features to the original Doom games, and, most prominently, includes the addition of loads of excessively gratuitous violence. While the original Doom is no slouch in this regard, with demons, lost of guns, viscera strew about levels, etc., Brutal Doom turns it up to 11. Enemies bleed excessively, their limbs can be individually removed, special execution animations have been added which allow the player to rip apart enemies with their bare hands, and much more.
Brutal Doom v20 will be released on June 5th. It requires you already own a version of classic Doom to work.