Okay I technically didn't find this in a darkroom, but...
I was visiting my parents today and they remembered an old pinhole camera i'd built when I was in high school. I used 4x5 paper negatives in it. We went looking for it and I ran across two half-gallon Delta 1 "Datatainer" bottles. They had who knows what in them. They were from 25+ years ago so I was sure they weren't any good, but figured I could rinse the bottles and use them once I get a darkroom set up to play around with pinhole photography again. One bottle had "fixer" wrote on it very faintly and when I poured it out smelled like fixer (it went down the drain because who knows how well 25-year-old fixer works). The other bottle was unlabeled and the liquid in it was bright red. I would assume it was developer of some sort, probably Dektol.
Incidentally the price tags were still on both bottles - $2.39. B&H still sells these for $3.95.
What sorts of things (that you may or may not have forgotten about) have you found in your darkroom?