Here are some Guitar Hero 1 Cheats I have found around the net. If you know of a Guitar Hero 1 Cheat that is not listed please leave it in a comment and I will add it. Enjoy!
Easter Egg Pushbutton Codes
Enter these at the Game mode selection menu:

Crowd has Monkey Heads:
Blue, Orange, Yellow, Yellow, Yellow, Blue, Orange

Crowd has Skull Heads:
Orange, Yellow, Blue, Blue, Orange, Yellow, Blue, Blue

Player uses Air Guitar:
Orange, Orange, Blue, Yellow, Orange

Rock meter will always stay green:
yellow, blue, orange, orange, blue, blue, yellow, orange

Unlock All:
Yellow, Orange, Blue, Blue, Orange, Yellow, Yellow

Unlocks Hero Guitar:
Blue, Orange, Yellow, Blue, Blue

Venue Disappears:
Blue, Yellow, Orange, Blue, Yellow, Orange

PAL Button Codes
Enter codes at main menu

Character plays Air Guitar (no guitar):
Yellow, Yellow, Orange, Blue, Yellow

Character plays Guitar Hero Guitar (SG Controller):
Blue, Yellow, Blue, Orange, Blue

Crowd cheat 1:
Yellow, blue, blue, orange, orange, blue, yellow

Crowd cheat 2:
Yellow, blue, yellow, yellow, blue, blue, orange, orange

No Venue mode:
Orange, yellow, orange, blue, blue, yellow

Rock meter stays green (practise mode):
Orange, blue, yellow, yellow, blue, blue, orange, yellow

Unlock All
Yellow, Orange, Orange, Yellow, Blue, Blue, Yellow