An ultracheap, 1/4 inch wide paintbrush type thing, whose sole purpose in life is painting on soldering flux, which is caustic greasy stuff that is otherwise a pain to work with.

Since they are ultra cheap (one of the few things you'll find at Home Depot in the 10¢ range), I imagine that they're intended to be discarded after use, but being somewhat of a cheapskate, I guess, I always lovingly squeeze the leftover flux out of the bristles with a paper towel, and store the thing away in my plumbing kit for next time...