Use these key combos at the extras menu to unlock the corresponding effect:
Cheat mode: Down, Right, Down, Right, Up, Down, Up, Left to unlock all single player levels, multi-player levels, options, and content
All multi-player mode skins: Down, Left, Up, Left, Right, Down, Left, Up
All artwork: Down, Right, Down, Right, Down, Up, Down(2), Right, Down, Right, Down, Up, Down, Left
Disable GoldenEye powers in multi-player mode: Up(2), Down, Left, Right(2), Left, Down
One Life mode: Left, Down, Up, Right, Up, Right, Left, Down
Paintball mode: Right, Left, Right, Left, Down, Down, Up(2)
Full health and armor: pause the game and press R(3), L, R(2), L, R
Infinite GoldenEye Power: pause the game and press L, R, L(3), R(2), L