Quadsaw Square Hole Cutter -- Quadsaw can cut a perfectly square hole in drywall/plasterboard in under ten seconds. The tool mounts to any drill that produces 1800rpms and it features 4 blades that buzz through the wall with precision & minimal dust. It is adjustable to also cut rectangular holes as well as adjustable depths.
Ruler Bits by mklotz -- I made this years ago for some long-forgotten project but it's proved useful a few times since then. Inevitably, there will be situations where you simply can't get a six inch scale 'in there' to measure something. My solution was to cut a scale into a number of pieces of different lengths and make a simple pinch clamp holder for the pieces. I wasn't doing anything requiring high precision so I used a plastic scale. I suppose one could make a precision version by...