Weeee let's have some fun with Ebay shall we?Nintendo World Championships NES Cartridge
Apparently there is only something like 120 of these carts in existence. They were used for the Nintendo World Championships (duh) and feature Super Mario Bros, Rad Racer and Tetris.
According to this auction the last time a cart like this went on auction it reached $2000 US. I tend to believe him from the quick research I did on the cart...
So yeah, go forth and prosper Ebay users... find weird and cool crap!