Latest gadget I have bought (2 weeks ago) ,but haven't had a chance to use yet, is Creative Grids Log Cabin Trim Tool. I watched a video on it and I love that the ONLY piece you must measure is the center one. AFter that, you call pull strips of odd sizes and simply trim them to the right size. Everything stays square. I bought the one that does both 6 and 12 inch in one ruler.
2 other gadgets I adore: the "Peels" that hug around your spool for storage. No more loose threads hanging off my thread rack. The other one is the little spring-loaded clamps called Tulips that keep thread on bobbins from getting loose in storage. I try to match peel color with hugger color and keep them on the same peg.
https://www.amazon.com/Prevents-Unwi.../dp/B00X037FGQ
https://www.amazon.com/control-Ergon.../dp/B00TTK44B8