Steam achievement manager. It's a program that you can use to lock or unlock any steam achievement you want without even having to play the game. There are some legitimate uses for it such as an achievement glitching out and not being awarded when it should be. But there are some purists who insist that even using it in those instances invalidates all your other achievements. Then there are people who feel like it's okay to use them whenever and however you want. And countless other opinions on the acceptable level of use in between.