Fallout 3 karma achievements and trophies

 are achievements and trophies which require you to have either bad, neutral or good karma upon reaching levels 8, 14 and 20.

Good Karma

 * Protector (level 8)
 * Ambassador of Peace (level 14)
 * Last, Best Hope of Humanity (level 20)

Neutral Karma

 * Mercenary (level 8)
 * Pinnacle of Survival (level 14)
 * Paradigm of Humanity (level 20)

Bad Karma

 * Reaver (level 8)
 * Harbinger of War (level 14)
 * Scourge of Humanity (level 20)

Strategies

 * An easy way to do this is continue the game as an evil person save at levels 7, 12, 18 (has to be near leveling up) and then when you re-load you just keep donating caps to the church in the game until desired karma level (If you don't have enough caps to raise your karma enough just sell all your stuff since it's not your main character). Then do anything to level up (for 12 and 18 just use the Here and Now perk to instantly raise your level when you reach 13 or 19). You can use the caps method to elevate your karma and simply execute one high exp rivet city guard, like the one standing out front to push yourself over the leveling line.
 * If your karma is Good (or very good) and you're close to a level for the achievement, fast travel to Megaton and start a killing spree. Pick off Stockholm with a Sniper Rifle from behind the pile of rocks on your left as you face the gate. As soon as he is down, Lucas Simms and Deputy Steel will come through the gate (Deputy Weld should already be out front as usual). You should be able to get the drop on Simms and pop him with the rifle before he can draw his weapon, leaving only the two droids to deal with. Go inside the gate and kill only as many citizens as necessary to drop your karma to neutral, then save the game, run out of town
 * Massacring rivet city can be more effective depending, it may be a tougher fight, but the guards give 50 points of experience per kill vs the measly amount given by megaton individuals. It may be a choice better for players further from their actual level up.
 * Also you could play as a neutral character and not complete the The Power of Atom Quest and then easily blow up or save Megaton with/from the bomb.
 * You can also make use of the here and now perk, as it can take you 2 levels in one level-up. If you have a save game at say late level 12, and not at 13, then modifying your karma and then leveling up from there with the perk will get you the achievements.
 * If you're close to a level, your karma is Good or Very Good, and Megaton has not been destroyed, there is a very quick and effective way to obtain all three achievements. You will need a Sniper Rifle, a very powerful weapon (I used the Firelance), and probably a lot of Stimpaks (I burned through about 20 of them to get Reaver). First, save your game before getting enough XP to level up (revert to a previous save if necessary), then fast travel to Megaton. Grab any equipment you think you'll need. (You might want to drop in to Craterside Supply for more ammo or Stimpaks. Sell anything off you don't want to use in the next thirty minutes or so; this is a very short-term strategy with an immediate reward.) Go outside the main gate of Megaton, where there is a very convenient sniping position behind some rocks on your right. Pop a couple of sniper rounds into Stockholm until he's down (he took two torso shots in VATS when I attempted this). Lucas Simms and Deputy Steel will come out of the gate. Take the two of them and Deputy Weld out (I was able to headshot Simms before he could even take out his weapon). Then head back inside town and start killing hostiles, while watching your karma meter. (Don't worry about hitting a "level up"; it seems as if the game won't actually advance you to the next level when there are hostiles around, and considering that you've just aggroed the entire town, this will certainly be the case.) I recommend using a weapon that can dispatch the hostiles as quickly as possible, as there are a lot of people in Megaton and some of them can be quite dangerous (Jericho (character) is nasty with his assault rifle, and some of the others are heavily armed as well). As soon as your karma drops to Neutral, save the game, run for the gate and keep going. Once you are far enough out of aggro range you'll hit your level up (provided you earned enough XP). Place your points anywhere, pick a random perk, and you should get your achievement. Now, load up your last saved game and kill enough people to drop your karma to evil and repeat the same process.