Uniball, which this month rebranded themselves Uni, makes a self-sharpening mechanical pencil called the Kuru Toga.
Here's how the self-sharpening system works. A gear system inside the barrel rotates the lead each time you press down.
Here's what that action looks like from the tip end. They've glued a piece of paper to the tip so you can better see what's happening.
So basically, it doesn't actually sharpen itself—but by constantly rotating the lead, it always presents you with a sharp tip, created by the last stroke you made on the paper, which wore one edge of the tip away. If that doesn't make sense, the video below explains it visually.