Big Head Mode:
Pause the game, press number lock, then hold "Numpad-7" and press C, B, Up, Left, Left, C, Right, Up, Left. If you entered the code correctly, the pause screen will shake. Alternately, successfully complete the game by completing all level objectives in career mode ten times.

Blood Mode:
Pause gameplay, press number lock, hold "Numpad-7" and press: Right, Up, C, V.

Cheat Mode:
Pause the game, press number lock, then hold Numpad 7 and press Spacebar, Spacebar, Spacebar, C, V, Up, Down, Left, Up, C, V, Spacebar, V, B, Spacebar, V, B. The pause screen will shake if you entered the code correctly. Next, select the "End Run" option from the pause menu. All cheats, FMV seqences, and skaters will be unlocked.

Fat Skater:
Pause gameplay, press number lock, hold "Numpad-7" and press: Spacebar, Spacebar, Spacebar, Spacebar, Left, Spacebar, Spacebar, Spacebar, Spacebar, Left, Spacebar, Spacebar, Spacebar, Spacebar, Left. Keep repeating to make skater fatter.

Neversoft Skaters
At the Main Menu, press number lock, hold "Numpad-7" and press: Up, C, C, V, Right, Up, B, V. Now create a skater with a Neversoft Dev Team name.

Raise Stats to 10:
Pause gameplay, press number lock, hold "Numpad-7" and press: Spacebar, V, B, C, V, Up, Down

Special Meter Always Full
Pause gameplay, press number lock, hold "Numpad-7" and press: Spacebar, V, B, B, Up, Left, V, C.

Thin Skater:
Pause gameplay, press number lock, Hold "Numpad-7" and press: Spacebar, Spacebar, Spacebar, Spacebar, C, Spacebar, Spacebar, Spacebar, Spacebar, C, Spacebar, Spacebar, Spacebar, Spacebar, C. Keep repeating to make skater thinner.

Turbo Mode:
Pause gameplay, press number lock, hold "Numpad-7" and press: Down, C, V, Right, Up, B, Down, C, V, Right, Up, B.

Jetpack Mode:
Pause gameplay, press number lock, hold "Numpad-7" and press Up, Up, Up, Up, C, V, Up ,Up ,Up ,Up ,C ,V ,Up ,Up ,Up ,Up. The screen will shake and Unpause and fly with ollie in mid-air to float. Use like the Playstation version.

Special Moves:
The first few moves are preformed when your character's special meter is flashing yellow.

Unlock Hidden Levels:
You can unlock secret levels (taken from the first Tony Hawk's Pro Skater) by earning a medal in the competitions. Earn the Chicago level in Marseille. Earn the Phoenix level at Skatestreet. Earn the Warehouse level at the Bullring.

Moves

Tony Hawk
The 900-Right, Down, C. 
Sacktap-Up, Down, C.
Christ Air-Left, Right, C.  
Varial-Right, Left, C.
Ollie north-Up, Up, B.
360 flip-Down, Down, B.
Layback Grind-Left, Right, V.

Bob Burnquist
FS Rocket Air Tailslide-Up, Down, V.
FS One Footed Smith-Right, Down, V.
Racket Air-Left, Down, C

Chad Muska
Backflip-Up, Down, C.
Sal flip-Up, Down, B.
540 flip-Down, Up, B.
360 shuvit-Up, Up, B.
360 flip-Down, Down, B.
Hurricane-Left, Right, V.
Nose Manual-Right, Up, V on the ground.

Geoff Rowley
Rowley Darkslide-Left, Right, V.
Double Hardflip-Right, Down, B.
Half Flip Casper-Right, Left, B.

Rodney Mullen 
Triple Kickflip-Left, Right, B.
540 Flip-Down, Up, B.
Darkslide -Left, Right, V.

Kareem Campbells Moves
Ghetto Bird-Down, Up, B.
Nosegrind to Pivot-Down, Up, B.
Casper-Left, Down, C.

Elissa Streamer 
Madonna Tailslide-Up, Left, V.
Hospital Flip-Left, Right, B.
Indy Frontflip-Down, Up, C.

Jamie Thomas
Laser-Flip Down, Right, B.
Secret Move-Down, Up, V.

Spider Man
Spidey Flip-Up, Down, C.
Spidey Grind-Left, Right, V. 
What Spidey Can Do-Left, Right, B.
 

Thanks to Revolution readers Daniel K. Browne and Joseph Tuvera!