These codes use the buttons on the guitar controller. Enter them at the main menu.
Master code (Unlock Everything):
Press Yellow, Orange, Blue, Blue, Orange, Yellow, Yellow. NOTE: The game can't be saved after this code is entered.
Rock meter always green:
Press Yellow, Blue, Orange, Orange, Blue, Blue, Yellow, Orange.
Press Orange, Orange, Blue, Yellow, Orange.
Toggle no venues:
Press Blue, Yellow, Orange, Blue, Yellow, Orange.
Press Blue, Orange, Yellow, Yellow, Yellow, Blue, Orange.
Press Orange, Yellow, Blue, Blue, Orange, Yellow, Blue, Blue.
Guitarist always plays Guitar Hero SG:
Press Blue, Orange, Yellow, Blue, Blue.
Guitar God Status:
Get a five star ranking for every song on every difficulty.
Playing Secret Characters glitch:
This glitch allows you to play unlocked characters from one difficulty setting in another. You must not have played the new difficulty ever! From the old difficulty level, select Izzy or Grim as your character, and allow the game to save. Then select your new difficulty and when prompted to select a character, just press (or Green). You'll notice that you'll be playing one of two secret characters. Be sure not to change your character, as you won't be able to get the secret characters back until they are unlocked in the new difficulty.
Thanks to Revolution readers TheLaw, xArchonx, ywangly and Thunderstrucker!