It's easy to underestimate the broad depth of the Super Smash Bros. franchise, but beneath the simple mechanics lies an incredibly in-depth meta-game and an huge community dedicated to playing and refining their smash skills. Some hardcore Smashers decided that the most recent effort, Brawl, didn't quite live up to expectations, so they set about tweaking the game. The end result? Project M.
Playing the game does not require any mods to the Wii itself, just a copy of Brawl and an SD card. Instructions on how to set this up can be found here.
For more info check out this incredibly detailed Neogaf post.
Thanks Anth0ny for the heads up!