Marvel vs. Capcom

Fight as Lillith in Morrigan's body:
Highlight Zangief at the character selection screen. Then, press Left(2), Down(2), Right(2), Up(2), Down(4), Left(2), Up(4), Right, Left, Down(4), Right(2), Up(4), Left(2), Down(4), Right, Down. She will appear below War Machine.
Information in this section was contributed by Daryx.

Fight as Roll:
Highlight Zangief at the character selection screen. Then, press Left(2), Down(2), Right(2), Down(2), Left(2), Up, Right, Up(2), Right(2). Roll will appear to the right of Mega Man.
Information in this section was contributed by Daryx.

Fight as Shadow Lady (Super Chun-Li):
Highlight Morrigan at the character selection screen. Then, press Up, Right(2), Down(4), Left(2), Up(4), Right(2), Left(2), Down(2), Right(2), Down(2), Left(2), Up(2), Right(2), Up(2), Left(2), Down(5). Shadow Lady will appear below Gambit.
Information in this section was contributed by Daryx.

Fight as Red Venom:
Highlight Chun-Li at the character selection screen. Then, press Right, Down(4), Left, Up(4), Right(2), Down(2), Left(2), Down(2), Right(2), Up(4), Left(2), Up. Red Venom will appear above Chun-Li.

Fight as Orange Hulk:
Highlight Chun-Li at the character selection screen. Then, press Right(2), Down(2), Left(2), Right(2), Down(2), Left(2), Up(4), Down(2), Right(2), Up(2), Down(4), Up(4), Left, Up. Orange Hulk will appear above Ryu.
Information in this section was contributed by Daryx.

Fight as Gold War Machine:
Highlight Zangief at the character selection screen. Then, press Left(2), Down(2), Right(2), Down(2), Left(2), Up(4), Right(2), Left(2), Down(4), Right(2), Up(2), Left(2), Down(2), Right(2), Up(4). Gold War Machine will appear above Zangief.
Information in this section was contributed by Daryx.

Change character order:
Press Strong Punch + Medium Punch + Weak Punch at the versus screen to switch the second and first characters.

Continue attacking:
Press Start immediately after winning the match to move almost everywhere and have the ability to continue hitting the defeated character, even with super moves.

Selecting special partners:
Note: The following codes must be entered while holding Start before the special partner selection screen appears.

Unknown soldier: Hold Weak Punch
Lou: Hold Medium Punch
Arthur: Hold Weak Punch + Medium Punch
Saki: Hold High Punch
Ton-Pooh: Hold Weak Punch + High Punch
Devilot: Hold Medium Punch
Anita: Hold Weak Punch + Medium Punch + High Punch
Pure and Fur: Weak Kick
Michelle Heart: Hold Weak Punch + Weak Kick
Mighty Thor: Hold Weak Kick + Medium Punch
Cyclops: Hold Weak Punch + Weak Kick + Medium Punch
Magneto: Hold Weak Kick + High Punch
Jubilee: Hold Weak Kick + Medium Punch + High Punch
Rogue: Hold Weak Punch + Weak Kick + Medium Punch + High Punch
Psylocke: Hold Medium Kick
Juggernaut: Hold Weak Punch + Medium Kick
Iceman: Hold Medium Punch + Medium Kick
Colossus: Hold Weak Punch + Medium Punch + Medium Kick
US Agent: Hold High Punch + Medium Kick
Shadow: Hold Weak Punch + Medium Kick + High Punch
Sentinel: Hold Medium Punch + Medium Kick + High Punch
Storm: Hold Weak Punch + Weak Kick + High Punch

Use opponent's helper:
Defeat your opponent's helper in a single player game. Then, hold Weak Kick + Medium Kick + High Kick at the screen that displays the name of the character that won until the next match. If done correctly, your helper in the last match will be from your previous opponent's team.

Name changes:
Enter --- as a name to have the game change it to "SIN". Enter AAA as a name to have the game change it to "CAP". Enter MMM as a name to have the game change it to "MOT".

Blow up Tyrant:
Note: You must have a level two super bar and Jill must be out for this trick. Use her Tyrant Super. Right before Tyrant leaves use her Rocket Launcher Super. If timed correctly you will blow up Tyrant in a very humorous fashion. Note: Blowing up Tyrant gives no extra bonuses, damage, or points.
Information in this section was contributed by Genki990.

Alternate costumes:
Press High Punch when selecting a fighter at the character selection screen.
Information in this section was contributed by hank.

Hint: Shadow helper:
If you look closely at the helper named Shadow, you may notice that it is actually Charlie from the Street Fighter series. When you call him, he will perform his "Shadow Justice" attack, then land for a moment. Look closely to see that it is him.

Hint: Ryu: Transformations:
When you reach one power bar press Back, Down, Forward + Medium Punch to transform into Ken. To transform into Akuma (Ryu in black), press Back, Down, Forward + High Punch (with one power bar). If you want to get back to Ryu after transforming to Akuma or Ken, press: Back, Down, Forward + Low Punch (with one power bar).
Information in this section was contributed by David Harari.

Hint: Hidden area:
On the level with one of the men drying off in the background, hit the wall on the right at least fifteen times and it will break and reveal a new room that you can fight in. This is done easier when you use any form of Spider-Man, Venom, or Carnage. Stand on the right side of the opponent then press the High/Fierce Punch rapidly with Venom or press the High/Roundhouse Kick with Spider-Man. [Screenshot] [Screenshot] [Screenshot]
Information in this section was contributed by Tan Nguyen.

Hint: Fight as Onslaught (Japanese version):
Highlight Zangief at the character selection screen. Hold Start and rotate the joystick twenty times in a clockwise circle before the timer expires, then press Low Punch + Medium Punch + High Punch + Low Kick + Medium Kick + High Kick.