Captain Falcon
Beat the game on any difficulty in UNDER 20 MINUTES. You must beat him to be able to play as him.
(Thanks to Kraken@pinkfloyd.com)

Item Menu
Play VS. mode about 100 times.
(Thanks to Kraken@pinkfloyd.com)

Jiggly Puff
Just beat the game and then you will face Jiggly Puff in a battle. You must beat Jiggly to be able to play as Jiggly.
(Thanks to Kraken@pinkfloyd.com)

Kirby Taunt
After eating another character Press L to do his taunt.
(Thanks to JaguarAdam1@aol.com)

Link and Double Hit
With Link, get out a bomb (down+B). Hold it for about 4-5 seconds and sword spin (up+B). It will take a while but will give your opponent an exploding sword spin.
Note: you will put a little damage on yourself.
(Thanks to Vegeto1991@aol.com)

Luigi
Pass the Bonus Practice 1 with at all the main characters. You can do this in practice mode. You must beat him to be able to play as him.
(Thanks to Kraken@pinkfloyd.com)

Mole Move
In the stages Kongo Jungle and Planet Zebes, you can perform the mole move. You do it by jumping underneath the main platform and then doing your Up + B moves. It will take you a while but once you learn it you will be able to do the mole on giant Donkey Kong and Samus Aran.
(Thanks to Vegeto1991@aol.com)

Mushroom Stage
Finish the game with all original characters . Mushroom stage will be available under versus mode.
(Thanks to JaguarAdam1@aol.com)

Music Test
Complete Bonus 1 & 2 with all characters, including secret ones.
(Thanks to Kraken@pinkfloyd.com)

Ness
Beat the one-player mode with any character, with the number of starting lives set to 3 and the difficulty set to NORMAL. You must beat the game without continuing. You must also beat him to be able to play as him.
(Thanks to Kraken@pinkfloyd.com)

New Outfits
At the character select screen press C-Left, C-Right, C-Up or C-Down.
(Thanks to JaguarAdam1@aol.com)

One Hit Knock Out
You ever wondered how to knock your best friend out of the level in just "ONE" hit!? Well here's a way to do it. First you start fighting, you see that weapons fall down every so often. Wait until you see a bat. Grab that bat, wait for your friend to come close and press >,>,A. Your guy will power up and knock him right out of the level. You can do this as long as you don't loose the bat. After that, you can laugh at him.
(Thanks to JaguarAdam1@aol.com)

Samus without Suit
Electrocute Samus with either Ness or Pikachu. Have the player controlling Samus press Start at the moment of impact. You perverts!
(Thanks to Littleman8230@aol.com)

Secret Level
Beat the game on normal with every character, than play in Vs. on every stage. You will get the Classic Mushroom Kingdom stage.
(Thanks to Kraken@pinkfloyd.com)

See Mew in the Pokeballs
Get all 4 secret characters to see Mew (randomly) in the Pokeballs. It has been reported you can get Jigglypuff and Luigi and see them too.
(Thanks to JaguarAdam1@aol.com)

Small Peach with Big Mushroom
Choose Peach and get a super mushroom, then pluck a plant out of the ground while you're still big, and when you get smaller, your plant will still be big and you'll be normal size.
(Thanks to ralila1@msn.com)

Sound test
Beat Board the Platforms and Break the Targets with every character.
(Thanks to Kraken@pinkfloyd.com)

Take Away Life
Right after you die on VS. Mode, press A+B+R+START and a life will be taken away from another player. However, this player must have at least 2 lives for the code to work.
(Thanks to Jkobrave@aol.com)

Taunt for Points
Use the L key to taunt your opponent right after he dies. If your character is taunting as the camera zooms you will get 100 points for the stance.
(Thanks to JaguarAdam1@aol.com)