Way more useful that I would have guessed.

Oscillating Multifunction Power Tool.

I needed to trim the shake siding around a window I recently installed to get the trim to fit (the new window was slightly larger than the old one.) I picked up the tool thinking it might work. Turns out it was just what I needed. I was able to zip off 1/2" around the perimeter of the window through a few layers of shingle and fiberboard insulation in about 30 minutes. Very impressive for a $20 investment.