Die Hard
     right, up, down, E                      god mode
     right, E, down, O                       50 grenades, smoke bombs, and more!
     right, up, down, down, E, right         change guns (repeat as necessary)

     Die Harder
     right, up, down, E                      change guns (repeat as necessary)
     right, up, down, E, E                   god mode
     right, E, left, O, A, down, E           extra missiles and grenades

     Die Hard with a Vengeance
     left, O, up, down, E, right             unlimited lives
 

Other Codes:
In any of the three games, hit Esc to pause the game. When the code is typed in correctly, the game should unpause.

     Die Hard
     right, E, E, down                       fat mode
     right, up, down, down, E, right         unlimited weapons

     Die Harder
     down, A, right, E                       god mode
     left, A, right, down                    keeps aim from automatically shifting
     down, E, A, down                        skeleton mode

     Die Hard with a Vengeance
     E, O, X, O, E, L, X, D, L               affects other cars once they are hit

Steve Smith