Monster Madness is the most popular of the mutators here. It adds monsters from Unreal’s single player game to any gametype. Killing them will give you a frag, killing a Titan or a Warlord will give you 3 frags because they are so much harder to kill.
There are a lot of configuration options, all accessable from a nice GUI inside the game. This includes the ability to select which monsters will show up.
This mutator was ranked a 9.3 of 10 by Modsquad.