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.
Shower mirror. It has a base that suctions to the wall and a reservoir that you fill with hot water so that it doesn't fog. I had no idea how much better it was than shaving at the sink. If I'm in a hurry I'll sink shave but I love shower shaving and I love that mirror.
Edit: Here's the one I use. No major complaints, just remove the mirror between uses and re-suction every once in a while.