Go to the "Cheat Codes" option under the "Options" menu, and type in the following as codes:

Cheat Code	Code
Low Gravity	getitup
Perfect Manuals	keepitsteady
Perfect rail balance	letitslide
Show all FMV sequences	digivid

Unlock Hidden Cowboy Character

Go to "Create-A-Skater" and change the name to Big Tex.

Unlock Various Bonuses

Complete all 129 goals to unlock Cool Specials, Kid Mode, Flame, Always Special, Perfect Skitch, and Rollerskates.

Unlock Gene Simmons

Beat Story Mode on Normal Difficulty.

Unlock Iron Man

Beat Story Mode on Beginner Difficulty.

Unlock T.H.U.D.

Beat Story Mode on Sick Difficulty.

Unlock Pedestrians

Clear every gap in the game.

Unlock Hotter Than Hell level

Complete the last goal (Kill Eric's Best Line). 

Unlock Create-A-Deck Mode

Beat Slamma Jamma in Story Mode

Unlock Bails 1 FMV Sequence

Collect 3 secret tapes.

Unlock Bails 2 FMV Sequence

Collect 6 secret tapes.

Unlock Always Hard FMV Sequence

Collect 9 secret tapes.

Unlock Lost Footage FMV Sequence

Beat the game under Too Easy and Beginner difficulties.

Unlock Alternate Ending

Beat Story Mode twice or Beat Sick Mode.