I bought a piece of 1.5 inch stiff foam to try to fix a sag in a bed. It didn't work but having that thick piece of solid foam around has been a life saver.
Need something flat to put a laptop on? Throw it on the foam. Going to be doing something that requires you to be on your knees for a while? Get the foam!
It went from stupid purchase to something I'd gladly replace if it broke.
I work in automotive interiors tooling. We had a customer bring by a sample of this headliner material. But they brought an entire roll of it, four feet wide, no idea how long. Probably 75 feet. I cut a couple feet off of it, now my whole desk is a mousepad.