Duke Nukem 64

At the title screen, enter any of the following codes:

In the cheat menu, key in R, C-right, right, L, C-left, left, C-right, right.

Enable the "Cheat mode" code. Press R, C-Right , Right, L, C-Left , Left, C-Right, Right at the main menu. In the UK version, press R, C-Right, Down, L, C-Up, Left, C-Right, Left. In the French version, press L, Left, Down, L, C-Up, Left, R, Left. In the Japanese version, press R, Right, C-Right, C-Left, L, C-Left, Right, Up.

Press Left(2), L(2), Right(2), Left(2) at the main menu to access the cheat menu. In the UK version, press Left, Down, L(2), Up, Right, Left, Up. In the French version, press Left, Down, L(2), Up, Right, C-Left, C-Up. In the Japanese version, press L(4), Right(2), Left(2).

First you need to enter the debug cheat at the press start screen, this is Up-C, Up, Left-C, Right, Down-C, Down, Right-C, R, A, B. Now start a multiplayer game with two or three players, not four. When you start use the spare pad to select a level and you will start a co-op game plus all the items will regenarate after a short amount of time.