I once found a SAK (don't know the model, it was a big one) on top of one of those chests of drawers at Home Depot (the ones the bolts and washers, etc are in). The main blade was open, like maybe someone had been using it to open boxes. I thought that maybe it might have belonged to an employee who put it there and forgot about it so I took it to the customer service counter, gave it to them and told them where I found it. Hopefully the owner got it back.
Another knife I found was a Buck Alpha Hunter folder (black rubber handles). I was riding to work when I passed something in the middle of the street that looked interesting. I did u-turn and it was a folder in a nylon web sheath. Stuck it in my saddlebag and went to work. At work I took it out to examine it, and although it had clearly been run over by a car, it still worked perfectly with no damage to any of the metal parts. The rubber handles had suffered some damage around the screws, and the snap on the sheath was wrecked, but other than that it was fine. A co-worker saw it and fell in love with it. So I told him that I would take it home and fix the handles and replace the snap. The next day when I gave him the knife he was like a kid on Christmas morning.