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.
A reusable floss tool. I was trying to cut down on waste by ditching those single-use floss picks. It's just a forked piece of plastic that you thread with standard floss. I bought it 5 years ago and it's still going strong!