Some time ago, mrmark0673 from the Digital Press Forum, found a working prototype of Hoppin Mad, an unreleased NES game developed by Elite. It seems that the game is a remake of a game that was previusly released on old PCs (C64, Spectrum, Amstrad), but with different levels. Later an article about the game was published on Nintendo Age with some better screens from the 3 playable levels.
If you would like to play the gam, you can download Hoppin Mad from here! Thanks a lot to Mr Mark for the contribution!
For more info: Unseen Interviews: Mr. Mark & Hoppin Mad