When I was a kid, I wore glasses, but my glasses were broken and being worked on, so I was without them. Still, was at a shopping center with my family and saw something on the ground, said "What's that?" and picked it up. It was a wallet with about $26 in it, iirc. We were right outside a store, so we left it there for a month, and no one claimed it, so I was able to get it afterwards.

When I was in college, I saw a $20 on the ground, and thought "Well, I don't need that, " and walked by. My girlfriend was like "Why not pick it up?" and I said it wasnt mine. A few seconds later, someone behind me saw it, grabbed it, and was very happy. I said I'd seen it, but not kept it. They said they were following the Grateful Dead (we were leaving a Dead concert at the time) and at a previous stop, the wind had come, blown a bunch of their cash away, and were down about $300. So we decided that this was the wind bringing some of their money back.

But really, if there's no way to know where it came from, it's yours now.