I just found a site that shows the photos they’ve found on film left in old cameras.  The “About” page says that he bought the film out of an old camera at an antique shop and then, years later, had it developed.  He only got 5 slides out of that roll but they were really good.  Now his site has hundreds of photos from the 50s, 60s, and 70s containing people he does not know but that are interesting pictures.  Check it out: Mangofalls.com