Lawbringer

Lawbringer is a Fallout 3 Perk.
 * Requirements: Level 14
 * Ranks: 1
 * Benefit: Once you have the Lawbringer perk, any evil character you kill will have a finger on their corpse. This finger can then be sold to a certain person (whose identity is disclosed when you take the perk) for caps and positive Karma.

According to an IGN interview, the "certain person" mentioned is a member of the Regulators who the player joins after taking the perk.