A prototype version of The Legend of Zelda—or Zelda no Densetsu, if you're a Famicom owner—has been released by the folks at The Lost Levels forums, a Nintendo curiosity that offers an early look at the NES classic.
This version of the original 8-bit Legend of Zelda was for the Famicom Disk System, the add-on for the Japanese version of the NES. The prototype release of Zelda features the kind of differences that only the most serious of Nintendo fans would probably care about, with altered dungeon layouts, non-finalized enemy designs and all kinds of prototype-y quirks.
It's a great opportunity to peek into the design process of this classic video game, even if many of the alterations will seem minor to some.
If this is the kind of thing you might want to pore over, the Cutting Room Floor has extensively cataloged the aspects that make this version of The Legend of Zelda different from its final release.
The Legend of Zelda prototype [Lost Levels Forums - thanks, B F!]