Sharpening stuff, as in different types of stones, sandpapers, grinders and jigs. I have a fairly large box of this stuff that is almost never used. I always looked for the next great sharpening craze. That all ended with the Worksharp 3000, between that for chisels and planes, the Drill Doctor for bits, and a regular 8" grinder for everything else I'm set. There's no more learning curve.