The fine folks at Rare Witch Project have finished reverse engineering the Goldeneye ROM enough so that you can design your own custom missions. You're basically free to use the game's resources in any way you could think of.
http://www.rarewitchproje.../specials.php?id=geeditorYou can either apply the changes to a ROM file and run it on an emulator, or if you have a gameshark with a parallel port, you can run it on the real N64.
All I have to say Is SWEET. I cant wait to use it.