Ready 2 Rumble Boxing

Champ class boxers:
Enter championship mode and enter CHAMP as a gym name. Exit championship mode and enter arcade mode to have access to all the boxers including Damien Black.

Gold class boxers:
Enter championship mode and enter GOLD as a gym name. Note: This also unlocks Nat Daddy in arcade mode.

Silver class boxers:
Enter championship mode and enter SILVER as a gym name. Note: This also unlocks Bruce Blade in arcade mode.

Bronze class boxers:
Enter championship mode and enter BRONZE as a gym name. Note: This also unlocks Kemo Claw in arcade mode.

Alternate costumes:
Press C-Up + C-Left at the character selection screen to change the boxer's outfit and pose.
Information in this section was contributed by Danceboy69.

Flurry mode:
Spell out RUMBLE, press A + B, and when your opponent is close press C-Down + C-Right.
Information in this section was contributed by CONMAN3365.

Fight with Tank Thrasher:
Quickly and repeatedly press C-Up + C-Down + C-Left + C-Right until Tank starts to make fun of his opponent.
Information in this section was contributed by Grouper44.

Taunts:
Press C-Left + C-Down or C-Up + C-Right during a match to perform a taunt.
Information in this section was contributed by BenMeh007.

Cheap vitamins and other items:
Choose to train your boxer In championship mode. Once in the menu, go to aerobics, press Left and immediately press A. If done correctly, you will have just purchased a $25,000 vitamin bottle for $500. You can do this with any of the training materials by pressing Left or Right then immediately pressing A before the screen stops on the desired item. However, buying the vitamin bottle will save the most money.
Information in this section was contributed by Danceboy69 and LCavitenia.

Choose to train your boxer In championship mode. Place the pointer on weight lifting training, which costs $5000, and move to the right. Press the A in-between the two items and you will purchase the vitamins or any other item for less money.
Information in this section was contributed by fRoSty.

Glitch: Game crash:
When you train a boxer to become a Champ, you will be taken to the "Select Boxer" menu to choose another boxer. If you press B, you will be taken back to the menu of the Champ boxer. If you choose "Title Fight", the game will crash, and you will lose all unsaved data.
Information in this section was contributed by AskPete.