Fallout 3 Karma

Karma is the reflection of all good and evil choices you make during the game and how they are perceived by the inhabitants of the wasteland.

Karma levels
You begin the game with a value of zero, which increases or decreases based on the players actions. There are 5 levels of karma:

Karma titles
Your karma status is defined by a "karma title", depending of your karma points and on your level. Karma titles above level 20 can only be reached with the add-on Broken Steel.

Positive

 * Donating caps to a church (In increments of 10, 50, or 100): +1 karma per cap
 * Selling fingers taken from the corpses of evil characters (you killed) to Sonora Cruz via the Lawbringer perk: +10 karma per finger
 * Giving away scrap metal to Walter in Water Processing Plant, Megaton for free: +10 karma per item
 * Giving away purified water to beggar characters like Carlos outside of Rivet City: +50 karma per item
 * Performing a good action in a quest: At least +50 karma (ex: Bring Cherry to Rivet City.)
 * Killing a very evil character or creature (Allistair Tenpenny, Mister Burke, Eulogy Jones, Carolina Red, Ahzrukhal, Jaime Palabras, Jotun, Ymir, and Cutter): +100 karma
 * Disarming the Megaton bomb in The Power of the Atom quest without requesting payment: +200 karma
 * Activating the failsafe in the Tranquility Lane quest: +300 karma
 * Activating Project Purity yourself in the quest Take it Back! : +1000 karma
 * Blowing up the Enclave Mobile Command Crawler in the Who Dares Wins quest in the Broken Steel add-on : +1000 karma
 * Taking the Escalator to Heaven perk in the Broken Steel add-on: +2000 karma (essentially, it resets your karma to the maximum 1000)

Neutral

 * Taking the Karmic Rebalance perk in the Broken Steel add-on instantly sets your karma back to 0.

Negative

 * Devour a corpse (Cannibal perk required): -1 karma per corpse
 * Stealing from a non-evil character or faction: -5 karma per instance (note that when stealing from owned (red text) containers, such as cabinets or lockers, the karma change applies once per activation of that container. Meaning, if you open a container once and steal two items, you lose 5 karma, but if you open the same container twice, stealing one item each time, you will lose 5 karma each time, which means you lose 10 karma). Items can be replaced in owned containers after being stolen, and then stolen again and again with the same karma loss each time, making for a useful method by which console gamers can decrease their karma, although repeatedly hacking an owned terminal is a faster way of doing this.
 * Repeatedly hacking an owned (red text) terminal (Classifies as stealing, but can be done over and over again) -5 karma per hack attempt, NOTE, even accessing the terminal will result in negative karma
 * Selling ears taken from the corpses of good characters (you killed) to Daniel Littlehorn via the Contract Killer Perk: -10 karma per ear
 * Killing a good creature: -25 karma
 * Performing an evil action in a quest: At least -50 karma
 * Killing a non-evil character: -100 karma (although, this can be gotten around by shooting them once, making them hostile, then letting a follower kill them.)
 * Giving psycho to Paulie Cantelli in Rivet City: -100 karma each time so long as he's alive
 * Enslaving any character with the Mesmetron: -100 karma
 * Causing Mister Lopez to commit suicide (either by pushing him or insulting him): -500 karma
 * Letting the feral ghouls into Tenpenny Tower in the Tenpenny Tower quest: -600 karma
 * Blowing up Megaton in the Power of the Atom quest: -1000 karma
 * Infecting Project Purity with the modified FEV in the Quest Take it Back!: -1000 karma
 * Blowing up the Citadel in the Who Dares Wins quest in the Broken Steel add-on : -1000 karma
 * Taking the Devil's Highway perk in the Broken Steel add-on: -2000 karma (essentially, it resets your karma to the minimum -1000)

Good karma

 * Access to Star Paladin Cross and Fawkes.
 * Talon Company Mercs will come after you.
 * A citizen in Megaton will reward you for your good-doings with ammo and other random gifts.

Neutral karma

 * The use of the Impartial Mediation perk, the single biggest skill boost in the game.
 * Access to Sergeant RL-3, a good early-to-mid game follower, and Butch.
 * Neither Talon Company, nor the Regulators will come after you.

Bad karma

 * The Regulators will be after you.
 * Access to Clover and Jericho, good early-to-mid game followers.
 * A slaver in Paradise Falls will reward you for your wrong-doings with ammo and other random gifts.

Companions and Karma
In Fallout 3, some companions have a karma level requirement that must be met if the player wishes to enlist them:
 * Clover and Jericho need a bad karma to be recruited. After being hired, they will follow the player regardless of changes in karma level, although they will often mention how boring the player is, should he become good.
 * Butch DeLoria and Sergeant RL-3, both recruited with neutral karma, are similar to Clover and Jericho. They will not leave if your karma becomes good or evil, but unlike Clover, you can't get them back with a high or low karma level when the main story separates you.
 * Players with high karma may hire Fawkes or Star Paladin Cross. They are not slaves or servants like Clover and Charon: they will follow you of their own free will, without any cost, if you ask them and have good karma. Be aware, however, that being good characters, they will leave your side if you become evil after hiring them, and will refuse to follow you again until your karma is high enough.
 * Dogmeat will follow you regardless of what your karma level is.

Sounds

 * Karma gain
 * Karma lose