Tony Hawk's Underground:
Cheat Codes: Enter the following codes in the cheat menu. 

Code - Effect 

moveinfast - Increased Speed
getitup - Moon Gravity
keepitsteady - Perfect Manual
letitslide - Perfect Rail Balance
digivid - Unlock all THUG Movies 

Unlock Cheats:
The following cheats become available at the cheats menu when you get 129 of 129 level objective goals: 

Always Special
Cool Specials
Flame Grinds
Kid Mode
Moon Gravity
Perfect Manual
Perfect Rail
Perfect Skitch
Rollerskates 


Hints
Unlock Civilian Skaters:
Get extra civilian skaters by finding all of the gaps in the game. 

Unlock Hidden Skaters:
Beat the game on the difficulty shown to unlock the following characters 

Gene Simmons - Complete Beginner Story Mode
Iron Man - Complete Normal Story Mode
T.H.U.D. - Complete Sick Story Mode 

Unlock More Hidden Skaters:
Enter the following as names for created skaters to unlock that skater. 

Name - Skater 

THEDOC - Adam Lippmann
Alan Flores - Alan Flores
Andy Marchel - Andy Marchel
Bailey - Bailey
Big Tex - Big Tex
fatass - Brad Bulkley
Akira2s - C Surla
arr - Captain Cody
Chauwa Steel - Chauwa Steel
ChrisP - Chris Peacock
Chris Rausch - Chris Rausch
Johnny Ow - Chum
Dan Nelson - Dan Nelson
Daddy Mac - Dana MacKenzie
DDT - Darren Thorne
TOPBLOKE - Dave Cowling
Dave Stohl - Dave Stohl
crom - Eric Grosser
Yawgurt - Gary Jesdanun
Glycerin - Glycerin
Greenie - Greeniev
grjost - grjost
GEIGER - Jake Geiger
Jason Uyeda - Jason Uyeda
NSJEFF - Jeff Morgan
Jeremy Anderson - Jeremy Anderson
Joel Jewett - Joel Jewett
FROGHAM - Kendall
Guilt Ladle - Kevin Mulhall
Marcos XK8R - Marcos XK8R
woodchuck - Michelle Deyo
Mike Ward - Mike Ward
moreuberthaned - Mikey Ortai
Noly - Nolan Nelson
POOPER - Paul Robinson
buffoon - Pete Day
deadendroad - Ralph D'Amato
M'YAK - Rock
1337 - Rulon Raymond
sik - sik
Skillzombie - Skillzombie
Stacey D - Stacey D
tao zheng - tao zheng
Hammer - Ted Barber
The Kraken - The Kraken
The Swink - The Swink
TSUEnami! - Todd Sue
Todd Wahoske - Todd Wahoske
leedsleedsleeds - Wardy
Y2KJ - Y2KJ
Zig - Zac Drake 

Unlock Alternate Ending:
Unlock an Alternate Ending by beating story mode twice on any difficulty. 

Unlock Cars:
Beat the following missions to unlock the car shown. 

Leafblower - Beat Either: "Race the blowcart" or "Ralphie got busted" in Vancouver
Limo - Beat the Mission: "Pick everyone up in the Limo" in Vancouver
Old Sedan - Beat the Mission: "Nut Vendors Unite!" in New York
Old Sedan - Beat the Mission: "Pineapple Killer" in Hawaii
Police Car - Beat the Mission: "Revenge!!!" in Tampa
Russian Sedan - Beat the Mission: "Party Favors" in Moscow
Security Cart - Beat the Mission: "No one likes a dude festival" in San Diego
Street Warriors Race Car - Beat the Mission: "Get in tight with the Street Warriors" in New Jersey 

Unlock KISS Movie, Concert, and Bonus Music:
Collect K-I-S-S twice in the "Hotter Than Hell" bonus level. Once to get the movie, and twice to get the ''concert'' and the two bonus KISS music tracks. 

Unlock Create-a-Deck:
Beat "Slamma Jamma" in Story Mode on any difficulty. 

Unlock Hidden Levels:
Hangar - In moscow, there is a red building at the start. Go to the opposite side of that building and walk into the fifth window from the left. 

Hotter than Hell - Complete Story Mode on any difficulty. 

School II from THPS 2 - Grab level icon from ledge above train station in New Jersey. 

Venice Beach level from THPS 2 - Grab level from inside tiki head in Hawaii. 

Unlock Movies:
Find secret tapes in the main story mode levels. 

Bails 1 Movie - Find 3 tapes
Bails 1 Movie - Find 3 tapes
Bails 2 - Find 6 tapes
Bails 2 - Find 6 tapes 
Always Hard - Find all 9 tapes
Always Hard - Find all 9 tapes