I carry a lot of gadgets when photographing in the field. A wooden spool with 1" gaffers' tape rolled around it, a dentist's mirror for checking levels and settings when I have the camera set up over my head, a Swiss army knife with the small screwdriver intentionally broken in half to make a smaller screwdriver, a mini-mag lite, an LED pocket light, 4-diopter reading glasses for focusing, etc. a retaining-ring wrench (fits in behind a 67mm filter in the filter case), a small roll of "skin tape" (light tack medical tape) for taping filters on lenses, a small tape measure for measuring bellows extension, 20' of nylon line and a couple of old bootlaces for tying things up/back, etc., a spare cable release, a large garbage bag, several twist-ties, a couple of micro-fiber cleaning cloths, a lipstick brush, and, one of my favorites, the no-longer available Voss filter holder with barn doors for a lens shade.
My Pentax digital spot meter and a Zone VI viewing filter are on lanyards attached to my fishing vest. They live in pockets, but the lanyards prevent them from hitting the ground if inadvertently dropped (which has saved the meter more than once).
My home-made darkcloth is made of white Gore-Tex lined with non-slip black cloth and equipped with Velcro strips. It will protect the camera from rain, mist and crashing breakers (and will function as a poncho for me when the camera's packed and it's raining). I also made a waterproof nylon drawstring bag that is big enough to go over the camera when set up. It doubles as an emergency bucket and, when filled with rocks, sand, etc. a good tripod weight.
I've got levels and scales on all my cameras, most of which I added myself. Stickers for true shutter speeds on each lensboard and for optimum f-stop calculation on each camera. My exposure record (small pocket-sized notebook) contains tables for reciprocity adjustments and bellows-extension factors for all the lenses I have as well as other exposure info (filter fudge-factors, etc.).
I've put hooks on the bottom of the center column or directly under the head on my tripods to allow me to hang my gear pack: a good way to weight the tripod as well as to keep the pack off the ground (I really hate having my pack on the ground!); it's also easier to work out of this way.
I find a couple of white cotton bandanas to be indispensable as well, for uses from everything to clean-up and hand-drying to putting on under my baseball cap to keep the back of my neck and ears from being sunburned. Wetting it before doing this is a real relief in the hot weather (I photograph in the deserts a lot).
Of course, I've got some business cards stashed away in an inside pocket somewhere too to hand out to curious/interested onlookers.
When on the road for a "photo safari," I have a bag of all sorts of other stuff in the car as well, everything from screwdriver sets to extra tripod leg-locks and knobs to a cigar end-cutter.... but that's for another thread
Best,
Doremus
www.DoremusScudder.com