Level Select and Invincibility
During gameplay, hold L+R+Y+Left (on pad) and press Start. A menu will pop up allowing you to become invincible or to warp to any level of your choice.

Play as Drizzt Do'Undren:
At the Character Select screen, hold L+R+Y+X and select any character. The game does not indicate any signs of the code working immediately, but in the mini-movie you'll see yourself as Drizzt. Alternately, beat the game on Extreme difficulty.

Unlock Gauntlet Mode and Extreme Difficulty:
Beat the game on any difficulty setting with any character to unlock The Gauntlet Mode. After defeating Gauntlet Mode you will open up the extreme difficulty level.

Tweak loading screen:
Use the Right Analog-stick to move the flames on the load screens.

Time Warp:
Whenever you fight a big boss that gives you lots of experience or you find a really good item, save before you find it and then when you get whatever you want save it on a different file. Reload the first one (before you got the item or experience) and then just import your guy from the second game (the one just after you got the item)

Thanks to Revolution readers Rafael Moriel, Andrew, Aaron Warner, chris ellis and Andres Zevgorba!