Super Mario Bros. Crossover Xplained: An Interview with the Creator

Clearly a labor of love, and very cool. But unless it's licensed, it's also a massive IP infringement. Some people think that if you don't make any money from it, it's not infringement, but that's not the case.

If I loved Zelda (and of course I do) and decided to recreate A Link to the Past and release it as a free iPhone game, I'm guessing I'd get a C&D in short order from Nintendo.