That big square ruler with the slits in it, it's still in its package and I don't remember how to use it or what it is for.
Anything that has a video about how to use it. If it needs that, its too complicated for me.
All of those little motoring tools, little funny shaped rulers that have special uses.
The pinking and wavy edge blades for rotary cutters.
If I ever get tempted by a notion, especially an expensive thing, I will look at this thread.