In Fallout 3 they are found on evil characters (such as raiders, slavers, smugglers, Enclave soldiers, and Talon Company mercs) when the Lone Wanderer has the Lawbringer perk. They have no weight, and while they have a value of 1, Sonora Cruz will reward you with 5 caps (10 if your Karma is very good) and +10 Karma for each finger. Any corpse of an evil character killed before you get this perk will not have a finger on it. If the evil character is killed by someone other than you (such as the Mysterious Stranger), or if you do not deliver the final shot the finger will not appear in their inventory. As long as you receive credit for the kill the finger should appear. Junders Plunkett drops a special finger that is worth 1,000 caps, and will only spawn once you have the perk. The evil Karma alternative of this Perk is the Contract Killer perk (where ears are found instead of fingers).
You must deliver the killing blow to the evil character for a finger to spawn. If any other damage source kills the enemy, be it another non-player character (including one of your followers) or even environmental damage, no finger will appear on the corpse.
A single finger may be found at an Enclave camp southwest of Vault 92, on a table, near many dead corpses in a truck. Without Lawbringer, this finger can do nothing.
Fingers also appear in the inventory of the patients you see who are unconscious lying on a bed in both Rivet City's and Megaton's clinic if they are killed and searched (despite that killing them will result in a loss of Karma).
To obtain the 5 caps bonus your Karma must be very good before delivering the fingers, therefore is advised that if you are carrying a large amount of fingers and/or your Karma is close to very good, drop a portion of the fingers before talking with Sonora Cruz (since you can’t choose the amount of fingers that you want to deliver), and deliver only enough fingers to get Karma to very good. Get the remaining fingers back, and deliver for 10 caps each.
pc Sonora Cruz can’t take more than 999 fingers at a time. If you have more than that in your inventory when delivering, she only takes 999 but still rewards you for all of the fingers (12,800 caps for 1,280 fingers with very good Karma, for instance). You can deliver the remaining fingers for more Karma and caps. [verified]