Loderunner 3D (N64)

Level Select:
Pause the game. Press and hold Z. Press R, B, A, B, A, C-Up, C-Down, C-Left, C-Right, C-Up, C-Down, C-Left, C-Right.

Bonus levels:
Pause teh game. Press and hold Z. Press C-Down, L, R, L, R, L, R, L, R, L, R.