Unlock everything...the easy way - To unlock all the cars, tracks, and game modes enter: X, X, Square, Square, Triangle, Triangle, O, O at the main menu. You will hear an engine rev if you did it right. NOTE: when you are entering the code you will go to different menus, don't worry about it, that's normal. 

Unlock everything...the hard way 
Beat Rookie, Get: 

Game Mode: Semi-Pro League 
Game Mode: Chase 
Car: Renagade 
Car: Mantis 
Beat Semi-Pro, Get: 
Game Mode: Pro League 
Game Mode: Chicken 
Car: Predator 
Track: Pelton Stadium 
Track: Meltdown 
Track: Hunter's Quarry 
Track: Scrap Heap 
Beat Pro, Get: 
Game Mode: Endurance Mode 
Track: Redneck Raceway 
Track: Chemical Plant 
Track: Go-Cart Carnage 
Beat Endurance, Get: 
All Reverse Tracks 
Car: Vandal 
Beat Arena, Get: 
Game Mode: Suicide 
Car: Widowmaker