Around 1992, Nintendo of America was all set to release Earthbound, the English version of Mother for the NES, but they stopped production at the last minute; I guess they didn't think it would sell too well due to its cumbersome gameplay. Anyway, one prototype cart of the game was kept. Several years later, it was auctioned off on EBay.
The winning bidder was some idiot who promptly posted a message on Usenet effectively stating that he would be happy to dump the cart into a ROM so it could be enjoyed by the entire Internet community... if he recieved $600. Even though this was an outrage, and even though he had no proof of actually having the cart or the equipment to needed make a ROM image, a bunch of Earthbound fans chipped in and mailed him the money. He did dump the cartridge (and later sold it), and the ROM is now widely available on emulation sites. The only hacking done to the original is the addition of the word ``Zero'' to the title screen and the fixing of a crash bug (possibly a form of copy protection).
As stated above, it's a pretty challenging game. I got pretty frustrated with it and haven't played in a while. It also has some bitchin' music.