Beat the game on any difficulty level to unlock the Gauntlet Mode.

Beat Gauntlet Mode to unlock Extreme Difficulty. This uses a character from a previous game.

Beat the game on Extreme Difficulty. Drizzt is now available as a player character.

Beat the game on any difficulty level to unlock the Elven Fighter as a playable character. The Elven Fighter is a combination of all three classes, having the strength of the fighter, the dexterity of the archer, and the spells of the wizard.

Have 2 players going at the same time, and go into town. One character drops whatever they want to duplicate. Import the character that just dropped their stuff - they will have all the things that they just dropped, plus the ones on the ground. Pick up the stuff, sell the duplicates, then save and repeat as necessary.

Right before the battle with Xanth, save your game. Defeat Xanth, then go back to the tavern and save your game in a different slot. Now reload the game you saved prior to the battle with Xanth, and select "Change character." Import the character from your save after the battle. Now go in and kill Xanth again for additional experience points. You can do this for any boss.

From within the game:
PS2: Hold R2 + L1 + Triangle + D-Pad Left.
Xbox: Hold down the left trigger all the way, the right trigger half-way, push the left thumbstick to the right, then push Y + A.
GameCube: Hold L + R + Y + Right.

If done correctly, your character will make a comment about cheating. If you are an Archer or a Mage, you will get all Archer Skills and Mage Spells. If you are a fighter, you will get all Mage Spells and Archer and Fighter skills. You can only increase stats for your character, so even though this will let a fighter cast Magic Missile, you can't increase the damage it does.

This is best done early in the game, since it sets all your skills to 1 point and your gold to 0.

PS2: Hold R2 + L1 + Triangle + D-Pad Left, then press Start.
Xbox: Hold down the left trigger all the way, the right trigger half-way, push the left thumbstick to the right, hold down Y + A, then press Start.
GameCube: Hold L + R + Y + Left and press Start.

This opens a cheat menu that allows you to toggle Invulnerability and warp to different levels. Xbox and PS2 versions allow warping between chapters, but GameCube is limited to the current chapter only. When warping between chapters, all level monsters respawn.

While in the game, press L1 + R2 + Triangle + D-Pad Left + R3 and Start. This will make your character 20th level, with 75000 gold, 1000 Health and 1000 Mana, and a minimum of 1 in every feat, including other character specific feats. Note that you cannot advance any further.

While in the game, press L + R + X + Right. This gives you 19,000 XP, 500 skill points, and all spells and feats.

PS2 and Xbox: Start Gauntlet Mode, then use the Level Warp cheat to go to the Tavern. You can now play the game as Drizzt, although you will not be able to use any weapons other than Twinkle and Icingdeath.
PS2 Method 2: At the main menu, press and hold L1 + R1, then push X + Triangle.
PS2 Method 3: Start a New Game and choose one of the 3 available characters. While the game is loading, hold R1 + R2 + L1 + L2 until loading is completed.
GameCube: Start a New Game and choose one of the 3 available characters. While the game is loading, hold L + R.

While there are no known cheats for this version, there are some definite exploits you can use to make things easier.

• Enemies will not follow you through doors. You can stand in a doorway and attack the enemy until they run away, run into the room to taunt them, then run back and repeat until they are all dead.
• Enemies seem to have a limited range, and will stop following you at a certain point. This allows you to rush in and attack, run away and heal, then attack again, or use ranged attacks without them fighting back.
• Enemies will often run into each other's line of fire. This is most helpful in the Thieves' Guild, where enemies will run directly in front of arrow traps. They will occasionally kill each other or themselves without you even lifting a finger.