Tony Hawk's Underground [GC]

Perfect Rail Balance
Go to the options menu and in the cheat codes area enter letitslide.

Go to options, then go to cheat codes. Then you can activate it from the cheat menu.
keepitsteady - Perfect Manual
letitslid - perfect rails
digivid - unlock all videos

Secret Characters
Iron Man - Beat the game on beginner.
Gene Simmons - Beat the game on normal.
T.H.U.D. - Beat the game on sick.
Pedestrians - Perform every gap.

Complete 129 of 129 Goals to unlock a full cheats listing
Cool Specials - Get 129 of 129 Goals
Kid Mode - Get 129 of 129 Goals
Rollerskates - Get 129 of 129 Goals
Flame Grinds - Get 129 of 129 Goals
Always Special - Get 129 of 129 Goals
Perfect Rail - Get 129 of 129 Goals
Perfect Skitch - Get 129 of 129 Goals
Perfect Manual - Get 129 of 129 Goals
Moon Gravity - Get 129 of 129 Goals

unlock hidden levels
Hotter than Hell - Complete Story Mode (on any difficulty level)
Venice - Go to Hawaii and go to the back left of the stage and ollie into the evil tiki statue
Hangar - Go to Moscow and turn right then go to the right side of the building and jump in the 5th window
School 2 - Go to New Jersey to the train station and it will be on the right side at the very top.