A bone folder.
Found: in my art supply box.
Use: scoring, folding, creasing, burnishing, accidentally puncturing, tearing, and scraping paper, cloth, and other materials for bookmaking, modeling, framing, wrapping presents,* etc. Also excellent for fidgeting.
Decision: I love this thing, and it’s shockingly useful. Put back in my art supply box.
*if you’re that kind of present wrapper…