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.
Bug zapper flyswatter. Like you can buy at Harbor Freight for a few bucks. It might not be a terribly effective solution to the overall fly population, but in terms of grim-bloody-vengeance-per-dollar, it's one of the best investments I've ever made.