It seems like this thread is more about the useful gadgets that we carry in the field than the actual cameras, lenses and filters, so I'll contribute in that vein. I have lots of little things that seem to make my life easier in the field and don't weigh too much. Here's a list.

1. A Voss "filter holder" with barn doors. Very small and compact. I use it for a folding lens shade and only rarely for holding a gel filter in a mount.
2. A compact flashlight. That plus the LED lights I usually carry in my pocket have got me out of many places after dark as well as help with focusing.
3. A homemade collapsible cover for the slide-end of filmholders. I made this from old film boxes. It just fits the protruding end of a filmholder with one darkslide pulled, and keeps stray light out. Really helpful for those long waits between shots when the wind is blowing and the darkslide whipping around...
4. A mechanic's flexible mirror. Like a dentist's mirror only larger and with a flexible head; for checking levels and shutter settings when the camera is too high or in other tight places.
5. A nylon "water bucket" with drawstrings. Originally intended for backpackers carrying water, it is large enough to cover my camera on the tripod to protect it from rain and it doubles as a nifty weight bag to hang from the tripod center column with a few stones in it. And it's reversible!
6. A small roll of nylon line and a few spare bootlaces. Indispensable for tying back pesky branches, anchoring the tripod, safety-tying the camera etc. Plus, I can replace the laces in my hiking boots if needed.
7. A lipstick brush and micro-fiber lens cleaning cloth. I use the brush a lot more than the cloth.
8. A roll of gaffer's tape. Actually 1" gaffer's tape wound around a small thread spool for everything from attaching filters to splinting sprained ankles.
9. A small level to augment the ones on my cameras.
10. A single lens magnifier that folds into a case to supplement my Peak loupe. A spare in case I lose the loupe or drop it off a cliff.
11. 4 diopter reading glasses for focusing. I use these regularly for rough focusing then use the loupe for fine focusing.
12. A Swiss army knife with one of the screwdrivers modified to be really small for all kinds of things.
13. A Zone VI viewing filter. Simply indispensable for me.
14. Lanyards for my light meter and viewing filter that attach to my jacket or vest. These have saved my spot meter more than once!
15. A couple of white bandanas for everything from lens cleaning to wearing under my cap to keep my neck from getting sunburned.
16. A flat lens wrench for tightening retaining rings. This just rides along with my 67mm filters and has come in handy once or twice.
17. A small measuring tape (I use a seamstresses retractable cloth tape when weight is an issue) for measuring bellows extension
18. Stickers for each lens that needs it with actual shutter speeds according to my shutter speed tester.
19. Stickers on each camera with optimal f-stops for focus spread. I use these always to get optimum DoF.
20. A couple of 30-gallon heavy duty garbage bags for rain protection, etc., etc.
21. My most important accessory: a small notebook with exposure records, tables for reciprocity failure adjustments and bellows extension factors for all the films and lenses I use plus lots of other useful information like exposure indices, Intermediate shutter speeds at longer times, etc., etc. With pencil, of course.

Plus I have a few stones, seashells and small pieces of interesting driftwood scattered around my photo vest pockets to remind me of places I've been.

In the winter, cold weather gear, liner gloves, etc. comes along too, but that's another thread, as is survival gear for real back-country hikes/backpacking.