Moveset swaps

A moveset swap is when hacking causes a character to take on the animations and moveset of another character. This is doable in both SSBM and  SSBB. Results vary; some characters can impersonate others with no trouble (most commonly clones or semi-clones such as Fox and Falco), while some combinations are unstable, freezing the game when some moves are used (or even as soon as the match is begun).

In Melee, moveswapped characters retain their body proportions and simply rotate their bones to match the new animations; for example, Donkey Kong's legs would pass through platforms if he was given the moveset of someone like Jigglypuff. However, in Brawl, characters change body shape to match their new animations, leading to unique distortions; for example, a character with short legs such as  Mario would have his limbs stretched if given the moveset of a tall character such as  Snake.

If the character used has loose hair, a dress, or the like, its will remain stiff if the animations do not include it. This extends to all parts of a character; for example Mario's ring fingers will remain "closed" when using Yoshi's three-fingered animations.

When a moveswap is used, most standard moves retain their characteristics from their original character. For example, Yoshi can use Luigi's forward smash without difficulty. However, more complicated moves may result in a "hybrid" move, such as using a Stall-Then-Fall when the original character does not have one. Special moves in particular have unpredictable effects, ranging from a surprisingly glitchless combination to a character-freezing situation (such as Mario, using Skull Bash, flying sideways at a constant speed until he dies).

In Melee, hitboxes would shift to accommodate the new moveset. In Brawl, hit boxes remain on the same body part. For example, Yoshi using Luigi's forward smash will still have the hitbox attached to his head (where it would be normally for Yoshi's forward smash).

Examples
These moveset swaps have gained a sort of popularity or recognition.

Pikaman (Pikachu as Ganondorf)
Pikaman, also known as "Pikadorf", "Ganonchu", "Longchu", "Freakachu", or "Manchu" (in reference to being manlike in appearance), is a moveset swap in which Pikachu is the character model and Ganondorf's moveset is used.

Manly-Man (Snake as Captain Falcon)
Manly-Man's moves cannot be used as it crashes the game. The name is derived from the popular meme that both Captain Falcon and Snake are incredibly manly, so a hybrid would result in a manlier man.

Mariolimar (Mario as Olimar)
Mariolimar, also known as "Mini-Mario" and "Olimario", is a moveset swap in which Mario is the character model and the moveset is Olimar's. He looks squashed, and tiny appearance, somewhat like a penguin.

Roadkill (Sonic as Fox)
Roadkill is the name of the moveset swap in which Sonic is the character model and Fox is the moveset. The name is derived from the fact that Sonic has his eyes closed the entire time and his arm in a fixed position, making it similar to "roadkill". For unknown reasons when Roadkill uses his Up Special Move it creates a golden Arwing in place of Sonic's spring. Note that other move swaps with Sonic can give him the appearence of having his eyes closed.

Wolvereach (Peach as Sonic)
Wolvereach, also known as "Screach/Screech" or "Nightmare Peach" is the moveset swap in which Peach is the character model and Sonic is the moveset. Wolvereach is the combination of the words "Wolverine" and "Peach" and is pronounced wolver-reach. The reason why this name was given to this hybrid is because of the long "claws".

Other Famous Moveset Swaps
As well as the given examples, several other moveset swaps have been seen. For example, one is of Toon Link given Metaknight's moveset, in which he loses most of his lower waist and down and walks around on two feet loosely connected to his upper waist. In another swap, Mario is given Luigi's moveset, making his nose smaller and face shriveled up. Mr.Game and Watch was given R.O.B.'s moveset making him look like a robot.

http://www.youtube.com/watch?v=NHxpnBGaJAE
 * A video of Moveset Swapping.