The ROM Hacker NesDraug (the creator of the excellent Beatles-themed Super Mario Bros. 2 hack Adventures in Pepperland) has recently released a new hack for The Legend of Zelda called The Legend of Super Mario, which transforms Hyrule into the Mushroom Kingdom and replaces Link with the mustachioed plumber.
The hack switches out the Triforce for 8 Starmen of Power and swaps the enemies and items in the overworld for Mario-themed equivalents. So rather than battling foes like octoroks, moblins, and peahats you'll instead have to defeat goombas, koopas, and snifits on your adventure to rescue Princess Peach.
Here's the full description of the hack taken from the project page:
"Welcome to Mushroom Kingdom, where Bowser has captured Princess Toadstool and thrown the Kingdom into tyranny. It is up to a heroic plumber named Mario to find all Starmen, become super and save the day!
Along the way you’ll be challenged by Goombas, Koopas and an endless array of ruthless creatures who will stop at nothing to prevent you from finding the 8 Starmen of power. Please hurry up and rescue Peach!"
Worth highlighting is that the download for the patch also comes with a handy manual featuring a map of the new overworld and a reference key for items and enemies. There's also a neat mockup for a Mario-themed take on the classic The Legend of Zelda box art too, should you want to display it among your other games. You can grab the patch here!