Donkey Kong Plus is the name of an unreleased game demo developed by Nintendo as a demonstration at E3 2002. Nintendo had planned for the game to be an extension / sequel of the Donkey Kong Game Boy game. The player would have the ability to design and create levels on the GameCube, and play them on the Game Boy Advance using the GameCube cable. The user-created level could be tested out on the GameCube, then saved on a memory card.
Although the game was never released, it did inspire the gameplay of Nintendo’s Mario vs. Donkey Kong, for Game Boy Advance. A level editor was not officially available in this game, but it was included in the sequel, Mario vs. Donkey Kong 2: March of the Minis for the Nintendo DS. [info from Wikipedia]
An hidden level editor was found by Martboo48 in Mario vs. Donkey Kong, but to unlock it you’ll have to edit one single byte in the games memory code. It’s unknown why Nintendo hidden this editor from the final game, but it seems that Donkey Kong Plus and Mario vs. Donkey Kong could really be seen as the same project.
Thanks to Hiccup for the contribution!