A long time ago, a hacker found that someone with a Wii could execute their own code using Legend of Zelda: TP. The hacker exploited a buffer overflow flaw in the game and in doing so, found he could run his their own game. Fast forward to now, and we see that a user by the name of goku64323 has managed to run the N64 version of Super Smash Bros on his Wii.
This is not the easiest thing to do in the world, it requires a lot of code modifications and a lot of praying that your Wii will not explode.
The hack itself isn’t free, you need to purchase “Sin and Punishment” from the Wii Shop Channel. This is because Sin and Punishment maxed out the game cartridge on the N64 (which coincidentally, so did Super Smash Bros).
Another person has laid out how he has hacked his games, if you are interested.
Anyways, it’s a pretty cool demonstration. Might want to try that for myself.