Regulators are a band of lawbringers lead by Sonora Cruz in Fallout 3. They dress in the Wild West fashion and wear clothes such as the Sheriff Duster and Sheriff Hat, and they operate in the Capital Wasteland. Their base of operations is an old farmhouse, named the Regulator HQ.
The Regulators will hunt the player down if your karma falls too low or if they destroy Megaton in the Power of the Atom quest as part of a random event. They will track the player down and attempt to kill you when you exit a metro station or fast travel. Encounters with them are random and can happen anywhere in the Wastes.