Man, I've found so much stuff I keep my eyes peeled now. In fact, about 2 days ago, while riding out in the country around here, I spotted a smart-phone hidden in the grass just off the pavement. I knew that had to have slid off a car roof or something, (which turned out to be the case). I turned around and grabbed it, the screen came right on, and when I stopped at the store where I top off my bottles, I started looking through recent calls to find someone to ring up. Long story short, that evening the girl's father came to get it, and even gave me a $20 gift card from the Dollar Store right up the road, LOL!!
I've found a couple single Allen wrenches, of the common bike size, so i assume they were lost by cyclists. I've found numerous other tools, (adjustable wrenches, screwdrivers, rolls of duct tape, etc), good work gloves, nice wool beanie cap (best one I own now). I once found a nice Columbia rain jacket, with a few bucks in the pocket, and business card for a hair salon on the other side of the county ( I called, but no joy). It was too small for me, so I gave it to my niece.
I also found a cool OD green Frog Toggs rain jacket, which I came back to at the end of the ride, and carried home (it's my car jacket now).
My mom needed a way to organize her snakes-nest of extension cords for her leaf blower and weed-eater, and all of a sudden, I spot one of those things that hold wrapped-up extension cords !!! The timing on that was downright weird !
I've found people's car keys, then posted them on Craigslist lost & found, but no luck there. Same with a kids baseball glove I found that had his name on it, I posted it, but no luck there,either.
Heck, I'm sure I'm still forgetting several things.

(edit: yep remembered another one! I once found a little nylon/velcro pouch that I believe attaches to the shoelaces of joggers, to carry a little cash and car/house key, and it did have several bucks in it.)
Years ago, my home-made bike rack in the garage needed some padding where there bike's top-tube rested across two 2x4's, and I happened to find about a 4-5 ft piece of foam pipe insulation. It may have fallen from a construction/plumbing truck, but it looked to be exactly what i needed. I folded it up across the mtn bike handlebar, and rode home with it, and it worked perfectly !