Enter the following at the password screen:

Alien craft: GIMME_DA_ALIEN (the underscores represent spaces)
All vehicles/levels: JTBT7CFD1LRMGW
All levels: LEVEL_SHORTCUT
All vehicles (except Alien): GANGS_UNLOCKED
Bonus Level: DDDDDDDDDDDDDD
Enhanced missiles: MISSILE_ATTACK
Hardest difficulty setting: I_AM_TOUGH_GUY
Hi-res mode: MAX_RESOLUTION
Invincibility: LIVING_FOREVER
Rapid-fire weapons: FIRE_NO_LIMITS
Reduced gravity: A_MOON_GETAWAY
Remove all enemies: POPULATION_OUT
Select same car: MIX_MATCH_CARS
Slow-motion gameplay: GO_REALLY_SLOW
View all endings: LONG_SLIDESHOW
Big Wheels: MONSTER_WHEELS