Pluggable modules for Unreal Tournament that slightly mutate gameplay. Mutators can be a lot of fun because you pick and choose what you like, and most mutators will operate in harmony with the rest.

All kinds of changes are possible. Changing weapons, ammo, gravity, speed, and game rules are just a few of the vast number of possibilities. Why not just use a mod? Mods have to be used exclusive of each other, so you can't pick and choose which rules you want to change. For example, you can't play Infiltration with Dark Magic weapons. You could do it if they were mutators.

Possibly the only failing of mutators is their authors' ambition; many mutators would be better off split up into smaller mutators and maybe parts of them made into a game type. Instead of choosing Infiltration weapons and then picking the weapons I want to deal with, I should be able to choose the weapons by implementing them one at a time as mutators. Then I could choose which weapons I want with no hassle, and I could play them with other mods.

Should maybe be called mutagens?