Sometimes the simplest things are the most genius. Case in point:
The picture placer (AKA a ruler with a screw in it).
It's so idiot-proof, they even printed the instructions on the placer itself, so you never have to worry about losing them:
Put the head of the screw under your picture hanging wire and hold the picture by the placer, like so:
Then press firmly on the center of the picture and the screw will make a tiny mark in the exact spot where you nail should go. Brilliant!
Why didn't I invent this apparatus? D'oh!
Originally spotted on Kelly + Olive.
Update: evidently you can buy a picture placer here.