There's a photographic accessory that I've used for years and consider vital, but I've never seen anyone else mention:
A hand towel is about the most massively useful thing a photographer can have. Partly it has great practical value - you can wrap it around your camera or lenses when putting them in an unpadded bag, or just have them loose as a buffer; put them on a rock or log for a place to sit while doing long exposures over brilliant marble-tile beaches; wrap it round your developing tank to ward off noxious fumes or to avoid dribbling spills; and of course dry your equipment off with it if it still seems to be clean enough.
More importantly, carrying a small towel has immense psychological value. Any photographer who can get though all of the electromechanical gear envy, rough it, slum it, struggle against terrible odds, get their photos ignored on social media, and still knows where their towel is is clearly a person to be reckoned with.
Seriously.
I have a small fleet of hand towels that I use for all of those things on a regular basis, and I almost always have one with me. It's a small thing – about a foot wide and two feet long – but so useful. They're also easy to find and cheap, and photography needs more things like that. Although I'm sure eventually someone will raise tens of thousands on Kickstarter with a 'photographic absorbent wrap', and fail to deliver two years later.
Hand towels are awesome.