The Legend of Zelda: A Link to the Past
Legend of Zelda:A Link to the Past has been extensively hacked.
Hyrule Magic is the principal Legend of Zelda: A Link to the Past editor. It is available here.
There is a very thorough Zelda 3 hack currently being developed by GameMakr24 called Zelda 3 Challenge: Quest for Calatia. When completed, this will be an entirely different game in the Zelda series, featuring new worlds, music, graphics, and dialog. GameMakr24 was previously responsible for the Zelda Challenge: Outlands, which did a similar feat for Zelda 1 and had over 130,000 downloads.