Bonus fighters:

Enter the following names in Career Mode. These can all be saved to your memory card for use in Career or Exhibition modes.
Ed Mahone
Gargoyle
Jermaine Dupri
Mark Ecko
Marlon Wayans
O
Q Tip
Tim Duncan
Roswell (Alien)
Shmacko (Clown)

Fight as Mills Lane:

Watch the "cyber athlete" video in the Options menu. Now start a fight in Exhibition mode and get disqualified from the match. Mills Lane will now be selectable as a middleweight.

Miniature boxers:

Watch the "cyber athlete" video in the Options menu. Then, start Career mode and enter MINIME as a name. To return to normal size, enter RESETPASS as a name in career mode.

Throbbing boxer heads:

In Career mode, enter the name THROB.

Illegal Kick in the Unmentionables:

During the fight if you push all four main buttons (triangle, square, circle and x) at the same time, your fighter will kick the other fighter right in the...err (well, think about it). You get an automatic point deduction for doing it, by the way.

Hints

Super Punch:

Press A + B + R and the direction to punch while the glove in the upper corner is flashing.

Super Heavyweight:

Go to Create mode. Create a Heavyweight and set his weight to 230. Push X to go to the next screen. Change his body type from "1" to "2". Now go back to the first screen. You are now able to change his weight from 230 to 280.