The most useless accessory I have spent my money on is a memory card wallet. I have NEVER used it. I'm glad it was cheap.
*shrugs* I use mine all the time. Keeps my flash cards from getting lost in my backpack. Then again, a plastic bag would do the same thing, for a few bucks less.
Tied with the "tripod" that has flexible legs and can supposedly wrap around a fence post or branch. (The brand name has something to do with primates.) Yeah, it might work with a smartphone attached, but not a dSLR of any size or brand.
I have one of those (don't remember which brand). It struggled to even support the weight of a subcompact camcorder. No, such hardware is worse than useless because it gives the false impression that it might actually work, then drops your camera in a smoking, $5,000 pile of glass and plastic....
I'd add all 'lenspens' to useless (or rather 'potentially dangerous') category. (A piece of microfiber fabric cleans a lens more efficiently. Or dry and clean t-shirt when I'm in hurry...)
Now there, I'm going to have to disagree with you. When I'm out shooting, I find that my shirts are anything but dry, and let's face it, a sweat-soaked shirt doesn't get finger grease off your lens very well. And microfiber cloths have to be washed, or they end up being greasy, and then they don't do any good.
Friends don't let friends buy Soft Focus filters.
I would put all
filters in that category, with the exception of clear and UV-blocking filters, and with the possible
exception of IR-pass filters. You can do pretty much anything else in post, but once you burn an effect into a photo, it's there forever....