I've always found the market for video game prototypes to be rather strange and fascinating all at once. With the recent attempted sale of a Final Fantasy II NES prototype getting attention for its $50k price tag, Boing Boing did some uncovering of some of the biggest sales.
My favorite rare game, for what it's worth, isn't a prototype at all but rather a gold cartridge considered the "holy grail" for collectors, the Nintendo World Championships game which was part of a massive tournament from Nintendo. The weirdest one I'm aware of is a Legend of Zelda title that was a downloadable title for a Japanese satellite system. All very strange.
Any old prototypes or lost games that tickle your fancy?