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've been bouncing back and forth between getting one, even though James and other people were initially like "this is dumb" but switched to "I love it". I just can't bring myself to spend $150 on a battery powered coffee mug, especially when I'm usually done with my coffee within a few minutes.