A really good pen.
Seriously leathermans are great, snapon screwdriver (multi bit) jewlers screwdrivers. small plier set. Snapon 1/4" drive - sockets can be anyones so long as multi hex (12 point). Tweezers, soldering tools. Good spanners, I use Stahlwillie cos they are slim and can get in most places. Problem with having multimeters, signal generators etc is that these require calibration stickers to work on aircraft (suppose to anyway).
