Codes
At the title menu, enter the code.
Cheat Effect
Orange, Blue, Orange, Yellow, Orange, Blue, Orange, Yellow Hyper Speed De/Activate
O,B,Y,Y,O,B,Y,Y Crowd has Monkey Heads
B,O,Y,O,Y,O,B Crowd has Eyeball Heads
O,Y,O,O,Y,O,Y,Y Flaming Head
B,O,O,B,O,O,BO,O,B Horse Head
Y,Y,B,O,Y,B Air Guitar
Yellow, Yellow, Blue, Yellow, Yellow, Orange, Yellow, Yellow Performance mode (Guitar God Mode?)
B,Y,O,R,O,Y,R,Y,R,Y,R,Y,R,Y Unlock All (NTSC Version)
PAL Codes
At the title menu, enter the code. Enter again to de-activate.
Cheat Effect
B, Y, O, B, Y, Y Air guitar
Y, Y, O, Y, O, O, Y, O Flaming Head
B, O, O, B, O, O, B, O, O, B, O, O, B, O, O, B, O Horse Head
Y, O, B, O, Y, O, B, O Hyper Speed
Y, Y, O, Y, Y, B, Y, Y Performance Mode (no HUD)
Y, Y, B, O, Y, Y, B, O Monkey Head Crowd
Y,O,B,O,B,O,Y Crowd has Eyeball Heads
Hyperspeed on Dualshock
Just do the sequence on the main menu (while selecting "Career Mode"). And keep doing it repeatedly until a message appears. Don't do it too slow.
Cheat Effect
Triangule, Square, X, Square, Triangule, Square, X, Square until a message appears
taken from: http://www.gamespot.com/ps2/puzzle/guitarhero2/hints.html?om_act=convert&om_clk=tabs&tag=tabs;cheats