I haven't lost anything REALLY expensive before, but have found 2 laptops, 1 in a cab and 1 in a cafe. I managed to get both returned.
The one I found in the cab when I sat down, luckily the last fare the guy had was to someone's home, so I had him take me there and we gave it to him on the way to my destination. Needless to say he was very grateful and had been calling the cab company but who really keeps track of what cab they were in?
The other one, I'm still a little miffed about. Found a laptop bag close to my seat in a cafe in my hometown. It was late and they were closing and the bag was still there so I collected it. Opening it up, I found a brand new laptop and no business cards or identifying info. I then booted up the computer hoping there would be some clue, but it was so new that Microsoft Word had never even been used! (It asked for my info when I booted it up) After a search through all the pockets, I found an invoice for what company had sold it, which I called and they managed to identify who had bought it but at an address 1500 miles away. After a few more phone calls I track down the person who had moved to my town and called him up to return it (turned out to be a MBA student). After all that, not even a word of thanks, just a "oh I was going to have my auntie buy me another one because I couldn't find this one!" *grumble*