Three LED (red, amber, green) over/under/just-right voltage indicator
which tells me if the alternator is on line. It takes up just 0.005
square inches of panel space.

A home-brew auto-muting stereo music switcher which added MP3/CD inputs
and stereo headphone jacks to my otherwise mono audio panel/mono
intercom. Auto mutes when there is activity on the aircraft radios or
intercom. Cost about $50 in parts, vs $1000 for the high end PS
engineering audio system.