At the Cheat code menu in options, enter the following:

Perfect rail balance:
tightrope

Unlock THPS1 Tony Hawk:
birdman

Unlock most features:
monkey c monkey do

Decreased gravity:
i can fly

The following skaters and levels will become unlocked after you perform the listed task:

Alien Doctor:
Beat Normal Story Mode

Australian Cop:
Beat Every Classic Mode Goal on Sick Difficulty

Boat Captain
Beat Normal Story Mode

Bratwurst Vendor:
Beat Every Classic Mode Goal on Sick Difficulty

Canada-THPS 3 Level:
Beat Normal Story Mode

Corn Vendor:
Beat Every Classic Mode Goal on Sick Difficulty

German Cop:
Beat Every Classic Mode Goal on Sick Difficulty

German Drunk 3:
Beat Every Classic Mode Goal on Sick Difficulty

Horn Player:
Beat Every Classic Mode Goal on Sick Difficulty

Lost Soul 1:
Beat Normal Story Mode

Lost Soul 2:
Beat Every Classic Mode Goal on Sick Difficulty

Metal Detector Guy:
Beat Every Classic Mode Goal on Sick Difficulty

Neversoft Video:
Beat Every Classic Mode Goal on Any Difficulty

Paulie Ryan:
Beat Normal Story Mode

Pirate:
Beat Every Classic Mode Goal on Sick Difficulty

School-THPS 1 Level:
Beat Normal Story Mode

Shrek:
Beat Easy Story Mode

Skatopia Punk:
Beat Every Classic Mode Goal on Sick Difficulty

Space Monkey:
Beat Easy Story Mode

The Hand:
Beat Normal Story Mode

Zombie 2:
Beat Every Classic Mode Goal on Sick Difficulty

Complete All Gaps in order to unlock the following:

All of the food venders
All of the musicians
Always Special
Bigfoot One (cartoony version)
Boxer
Inline Skater
Lost Soul 3
lounge singer(the devil)
Mayan with Tiki mask
Moon Gravity
Perfect Grind
Perfect Manual
Perfect Skitch
Skaboto(robot in front of skatopia)
Space Monkey 3


Cheat: Single Disc Mulitplayer

If you want to play against friends, but only have one disc, there's a way you can still play using one UMD. Create a game on Ad Hoc mode and start the game on one PSP, wait for the game to load, then remove the UMD. Place the game in another PSP, load the game, and there you have an available multiplayer game!