I found a piece of 5/8'' hot rolled plate laying on the berm of the road. It is 6" by 48" Not rusty. Last autumn I found a like-new 5 gal Eagle metal safety gas can with the attached funnel laying partially covered with leaves about 6 feet off the road in the woods.
Although not strictly a roadside find, my best unexpected yard sale find was someone whose father was a machinist and long gone. The person didn't have much of interest at the yard sale, but he mentioned his father's tools when we were talking and then took me down to his cellar to see them. I bought a bunch of stuff at almost give away prices. e.g. an Interapid DTI for either $15 or $20, I forget now. The tools had been stored in a damp basement and many had a light coat of rust. I was hesitant to pay a whole lot for many of them since I didn't know how the rust would affect the accuracy of some things. In the end I made out pretty well.