At the title menu, enter the code.

Code Effect

Y,Y,B,O,Y,B Air Guitar
B,O,Y,O,Y,O,B Crowd has Eyeball Heads
O,B,Y,Y,O,B,Y,Y Crowd has Monkey Heads
O,Y,O,O,Y,O,Y,Y Flaming Head
B,O,O,B,O,O,BO,O,B Horse Head
Orange, Blue, Orange, Yellow, Orange, Blue, Orange, Yellow Hyper Speed De/Activate
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)

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.

Code Effect
Triangule, Square, X, Square, Triangule, Square, X, Square until a message appears Activates the Hyperspeed

PAL Codes - At the title menu, enter the code. Enter again to de-activate.

Code Effect

B, Y, O, B, Y, Y Air guitar
Y,O,B,O,B,O,Y Crowd has Eyeball Heads
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, B, O, Y, Y, B, O Monkey Head Crowd
Y, Y, O, Y, Y, B, Y, Y Performance Mode (no HUD)

Unlockables

New multiplayer mode - Pro face-off mode is where both players play all the notes.

Pro Face-off Beat game on medium or higher

Secret Guitars - Complete the designated in-game tasks to unlock these secret guitars in the shop.

"Snaketapus" Guitar Beat Hard Mode
"The Log" Guitar 5-Star every song on Expert
Battle Axe Guitar Beat Expert Mode
Casket Guitar Beat Medium Mode
Eyeball Guitar 5-Star every song on Hard
Fish Guitar Beat Easy Mode
USA Guitar 5-Star every song on Easy
Viking Guitar 5-Star every song on Medium

Here is how you can unlock additional Basses in Co-Op mode.

Cream SG Get 5 stars on 20 songs in Co-Op

Gibson Grabber Beat 20 Songs in Co-Op Mode
Gibson SG Bass Beat 10 Songs in Co-Op mode
Gibson Thunderbird Beat 30 Songs in Co-Op mode

Hofner Bass Beat every song on Co-Op mode

Lava Pearl Musicman Stingray Get 5 stars on 10 songs in Co-Op

Natural Maple Gibson Grabber Bass Get 4 stars on all co-op songs.
Natural Sunburst Gibson Thunderbird Bass Get 5 stars on every song in Co-Op