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.

Bugs
The following may not be applicable for all versions and all platforms.

It is possible to lose Sonora Cruz and never be able to give any more fingers to her. Fast traveling to the Regulator HQ will often spawn either two Yao Guai, two Giant Radscorpion or some combination. They will be busy killing the Brahmin outside the HQ, however they will engage you upon your arrival. Should you opt to run past the creatures and into the Regulator HQ without killing them first, Sonora Cruz will run out of the Regulator HQ building (supposedly to help fight with you). If you follow her outside she will not be out there. Searching the area for her body is fruitless and she will never return to the Regulator HQ.

Sonora hasn't disappeared, she is actually fleeing. You can usually find her in the surrounding area of the Regulator HQ. Just make sure you have killed any hostiles before you try and find her or she will run off again. If there are no hostiles to be seen and she is still fleeing, try attacking her (doesn't give negative Karma) until she falls unconscious. When she wakes up she won't attack and you can talk to her as normal.

For those ambitious types: If you can collect 1000 or more fingers there is a glitch you can cash in on. When selling Sonora any quantity of fingers above 1000, she will pay you the full amount but only subtract 999 fingers from your inventory, leaving you with the quantity of fingers of X-999 in your inventory. You can then sell her the fingers "again". So if you were to sell 1200 fingers to Sonora she would give you 12,000 caps and you will still have 201 fingers left in your inventory to sell again, resulting in another 2010 caps profit.