Remember . . . if it has an asterisk (*) beside it, that means I have personally checked that code out and it works. If there is no asterisk, then it is a code that I have neither checked out nor disproven. You're on your own with them.
*Dr. Robotnik
Code - Place First in Radiant Emerald course.
Affect - Make Robotnik a playable character.
*Secret Characters
Code - Gather all five Sonic tokens on each course (except
Radiant Emerald) and place in the top three, then defeat the secret
character.
Affects - Makes the defeated character playable. Characters
are found as follows. Resort Island - Mecha-Sonic. Radical City - Tails
Doll. Regal Ruin - Egg Robo. Reactive Factory - Mecha-Knuckles.
*Super Sonic
Code - Find all the Chaos Emeralds.
Affects - Makes Super Sonic a playable character. Press up
and down during player selection to switch between Sonic and Super Sonic.
Chaos Emeralds are distributed as follows. 1 in Resort Island, 2 in
Radical City, 2 in Regal Ruins, and 2 in Reactive Factory.
*Spin Dash
Code - Hold down and rapidly press B just before the race
begins. Release at "GO!".
Affects - Player will get a short boost and gain a head start.
This only works for some characters.
*Clean Pause
Code - Press X, Y, and Z at the same time while game is
paused.
Affects - The "Continue, Retire, Retry" menu will disappear.
*Zoom
Code - During game/player selection, press the R or L button.
Affects - Makes the game/player icon zoom in and out.
*Title Hijinx
Code - Wait a few seconds when the title screen appears.
Simple enough, yes?
Affects - A button turns the R red. B turns it metallic. X
darkens it, Y lightens it. The directional pad makes it swing up and down
and side to side and dosey doe.