Multi-player Hack in Legend of Zelda: Ocarina of Time - video powered by Metacafe
Reportedly from the same esteemed programmers who brought you the Arwing and Dark Link hacks in The Legend of Zelda: Ocarina of Time comes what might be the magnum opus of N64 cart mods. I won't believe it until I play it myself, but we apparently have reason to believe that some insane Four Swords-style action will be coming to Ocarina in the near future.
This is the kind of modification Nintendo should be making themselves, preferably to their Virtual Console releases on the Wii. Between this and the Mother 3 translation hack, it seems like everywhere you look, Nintendo fans are working on Nintendo games in ways that Nintendo themselves just won't do.
You can thank me for not doing a tired "Ninten-don't" joke later.