All characters and levels

Go to the Options menu and choose "Game Status". Highlight any driver and press Circle (passcode), then enter the passcode "WMNNWLHTSCUCLH". This will give you access to all secret characters, levels, and 'Y' the Alien.

Invincibility:
Enter "I_WILL_NOT_DIE" as a passcode.

No enemies:
Enter "GO_SIGHTSEEING" as a passcode.

Low gravity:
Enter "REDUCE_GRAVITY" as a passcode.

View FMV sequences:
Enter "SEE_ALL_MOVIES" as a passcode.

Expert mode
Enter "HARDEST_OF_ALL" as a passcode.

Same vehicles in two player mode:
Enter "SAME_CHARACTER" as a passcode. This will make it so both players can choose the same vehicles in two player mode.

Monster truck tires
Enter "MONSTER_WHEELS" as a passcode. This will make your tires big.

More powerful missiles
Enter "DEADLY_MISSILE" as a passcode. This will make your homing missiles do more damage.

Secret weapon attacks
Each weapon has two secret moves. The manual tells you one and these are the other ones. Note: These special attacks will still work even if the weapon, is not your selected weapon.

Interceptor Missiles: Halo Decoy
Up, Up, Down, Fire Machine .
This will cause any homing missiles or special to follow your missiles instead of you. Useful but not easy to pull off. This is one of the only ways to cause the Gamma Storm from Beezwax to miss. This attack takes 2 missiles away from your inventory.

Bull's Eye Rockets: Stampede
Up, Down, Up, Fire Machine .
This shoots up to 5 of your rockets in a quick secession. They don't aim automatically so most will miss unless your target is stationary: I.E. stalled or in a Bear Hug Mine. Also using this while spinning around is great for multiple enemies. This attack takes 1-5 rockets away from your inventory.

Sky Hammer Mortar: Turtle Turnover
Down, Down, Down, Fire Machine .
This will cause the opponents car to flip over on it's roof. The computer flips right back up. But some of the bigger cars take a while so it can be useful. This attack takes 2 shells away from your inventory.

Bruiser Cannon: Cow Puncher
Down, Up, Down, Fire Machine .
This not only does a lot of damage in one hit, but it also throws the opponent for a loop. Well, it just pushes them back and sometimes will tip them over. Great fun if your opponent is on the side of a cliff. This attack takes 2 shells away from your inventory. Roadkill Mines: Cactus Patch
Left, Right, Up, Fire Machine .
This will cause you to drop up to 6 mines in a area behind your car. If someone is following you they are guaranteed to hit at least one of them. And the best part is that unlike Twisted Metal 2, mines stay where you put them. So be careful! This attack takes 1-6 Mines away from your inventory