I get the shipping cartons for blueberries and strawberries in the box bin at Costco. They are relatively flat--4 or 5" high and they stack. I can stack 6 or 7 high and have a separate layer for projects, thread sets, etc.
I put a memory foam bath mat under my machine. It absorbs vibration and is a great pincushion.
Men's trimming electric razors (or a hairdressers clipper with a 40 or 50 blade) can be used to frog (same as Peggy's Eraser) but lots cheaper.
I save clear packaging from who knows what to use with a Sharpie to trace designs. Much like using a photo album clear page.
Swimming pool noodles to wrap up finished quilts to take to show-and-tell. Wrap with selvedges.
Tyvek envelopes or Tyvek from the dumpster of a house under construction for art quilts. Paint on it with luminere paint and use a heat gun to scrunch it!
Magic Eraser to clean an iron.
Coloring books to get "cheater" designs for appliqué or art quilts---especially Dinosaurs!!!
Turkey baster to squirt water into iron…or a 2oz syringe!