If the player has the Lawbringerperk, he/she can trade fingers taken from the corpses of evil humans or (non-feral) ghouls killed by the player, for good karma and 5 to 10 bottle caps depending on karma level. The Regulators will also attempt to claim the bounty on the player's head if he or she has bad karma by committing serious crimes, such as destroying Megaton or enslaving people in the Capital Wasteland, or by simply murdering people.
Regulator is the actual term for an official who maintains control and supervision of an area of public interest.  As a result, many armed groups, both vigilante and official, have been named "Regulators" throughout American history. One of the most notable groups in American history were the Regulators of the Carolinas around the time of the American Revolution.
↑Fallout 3 Official Game Guide Game of the Year Edition p.97: "Sonora Cruz Leader of a hidden organization known as the Regulators (who are recognized by their long overcoats known as "dusters"), Sonora Cruz is a woman with a mission: to rid the Wasteland of troublemakers and ne'e-do-wellsQuoted verbatim, error appeared in the original source. Her unknown benefactors, who set up this clandestine operation around a decade ago, are willing to pay for scalps, or in the Regulators' case, fingers." (Fallout 3 Official Game Guide Game of the Year Edition Wasteland Census)
↑Fallout 3 Official Game Guide Game of the Year Edition p.82: "Lucas Simms He grew up in Megaton and understands better than anyone what it takes to survive and thrive in the Wasteland. Big and imposing yet warm and friendly, Lucas won his position by a unanimous vote of the people of Megaton (most of whom genuinely love him). He serves as mayor, sheriff, judge and just about any other position that's needed. It's also no secret around Megaton that Lucas Simms is fulfilling some weird childhood cowboy fantasy. He named his son Harden after the Old West outlaw John Wesley Harden. Simms is also a member of a secret society known only as "the Regulators."" (Fallout 3 Official Game Guide Game of the Year Edition Wasteland Census)