Here are some cool cheats:

Miniature Boxers
Enter the options screen and view the Cyber Athlete video sequence. Then, enter career mode and enter MINIME as a name to miniaturize all boxers. To return to normal size, enter RESETPASS as a name in career mode.

Throbbing Boxer Heads
Enter THROB as a name at the boxer creation screen.

Fight as Judge Mills Lane
Enter the options screen and view the Cyber Athlete video sequence. Then, enter exhibition mode and intentionally get disqualified. Mills Lane will be unlocked as a middle weight boxer.

Fight as Jermiane Dupri
Enter JERMAINE DUPRI as a name at the boxer creation screen to fight as Jermiane Dupri, the hip-hop musician. Press Circle at the pre-fight ranking screen to save the boxer for use in career and exhibition modes.