I suggest you go here:
kitty corps meow mix and check out the forums
Edit: I might as well just quickly tell you some of the things.
You may now change character models (not very easy though) and import some models from other games.
You can make your own move-sets for characters.
You can change/add models onto stages and change the collisions.
You can change menu images and how characters are ordered in the character selection screen and also change the portraits.
Brawl hacking is awesome, if only it was easier.