Finish Simple (Classic) Mode on any
difficulty setting without losing a life as Mario.

Note: Dr. Mario is a Mario clone just with different looks...in other words he has the same moves except for his b attack...in which normally if playing as Mario when you press B he'll shoot a fireball. While playing as Dr. Mario he'll throw a pill instead.

Unlock Falco

Finish the 100 Man Spar mode (under Colesseum.)

Note: Falco, is to Fox as Dr. Mario is to Mario. In other words, Falco is a clone of Fox...he has all the same moves except for his B attack. Falco's B attack is just a stronger version of Fox's B attack.

Unlock Gannondorf

The only requirement is that you clear Event 29 in Event Mode.  This requires you to beat some other events.  Just keep beating events until 29 is playable. In event 29 you, Link, and your ally, Zelda, try to beat Ganondorf.  IT is rather hard if you are not used to Link, but don't give up. 

Also reported as:

Complete Event Mode No. 29

Note:Gannondorf isnt an exact copy of Captain Falcon but his moves are sorta based on C. Falcons moves. However they are much stronger

Unlock Jigglypuff

Simply beat the game...with any character, any difficulty setting and continues can be used.

Note: Jigglypuff's moves are the same as they were in SSB

Unlock Luigi

Beat Stage One of Adventure mode with a time in which the number of seconds ends in a 2- xx:x2:xx.  One example is with 4:12:33 seconds left before time runs out.  On the next stage the game will play a scene of Luigi jumping off Mario's head to get into the fight.  Beat him and Peach in under 1 minute.  Clear the rest of the adventure mode and Luigi will challenge you.  Defeat him to unlock him.

Also reported as:

Complete Adventure Mode as Pikachu. Pikachu's box on the select screen will be pushed down and will occupy a ? box. Luigi will take his place on the select screen.

Note: Luigi's moves are different from the way they were in SSB but his B moves are pretty much the same.

Unlock Mewtwo-(This ones going to take a lot of time and alot  of aspirin...)

Play VS mode 700 times or play VS mode for 20 hours

Note: Mewtwo has his own set of moves:

Short Jab - A
Tail Swipe - Forward + A
Psionic Fireball - Smash A
Low tail Swipe - Down + A
Psionic Charge - Smash down A
Samus Style energy charge - B
Flip opponent when close - Forward B
Dizzy Stun - Down B
Teleport - Up B

Unlock Mr. Game & Watch

Beat Adventure or Classic Mode with all 24 characters.  You can use any level, any amount of stock, and unlimited continues.

Also reported as:

Complete Simple (Classic) Mode with every default character (that's 14 of them). Mr. Game & Watch will challenge you. Defeat him to play as him.

Unlock Young Link

Young Link is the same as Link except he has a higher voice, so don't kill yourself trying to get this one.  Beat classic mode with ten characters.  Link and Zelda must be among the ten characters.

Note: Yep you guessed it; an adult Link clone

Easy Roy and Marth trick

First Go to vs. mode Put cpu as Yoshi with no handicap then Put stock on only one life. Now start with the person you're worst with and kill yoshi with every character (best person last) Marth will challenge you win you have Marth. Now Go to Classic And use Marth don't die once and Roy will challenge you win and (Roy is awesome) you now have him.

Unlock Pichu

You must first unlock both Luigi and Falco. Now, go and complete Event #37. Once winning, Pichu will become playable.

Note:For 500 dollars what is pichu?????
You got it!!! A pikachu clone.

Unlock Marth

Play 70 rounds in VS. Mode

Beat Classic Mode with all 14 DEFAULT characters .

Unlock Roy

Beat Classic mode without losing a life with Marth.  Also reported as: 

Note: Roy is a Prince Marth clone except his moves use the element of fire in all of them.