A medium-soft toothbrush is a vital tool in cleaning synths and music gear. It can remove dust from grooves in amplifier vents, in faceplate corners and inner edges, and if you get an old piece with dirty knobs, is perfect for lightly scrubbing them off (after removing them from said gear). It can also remove dust and dirt from under knobs (with a bit of water, just slightly damp) and all edges of gear and stereo equipment. Makes things look a lot newer.
Some use them to scrub PCBs after defluxing.