When you start the game, load a saved city or start a new city (it doesn't matter). As soon as the game loads your city, go to 'Save / Load' and select 'End'. Then when you are at the screen that says 'See you soon! Goodbye!', input the following code on THE 2ND CONTROLLER (it will not work on the 1st controller):
Left, A, Right, Y, Up, B, Down, X, Select, Start, Start, Select, R (twice), L (twice). A menu should pop up with the following options:
Sound Test - plays every background music and sound effect in the game.
No Disasters - disables the ability to cause disasters and almost completely disables disasters during scenarios. Plane crashes and nuclear disasters can occur if your city is set on Hard mode.
Needless Money - This allows you to build anything you want for free, regardless of how much money you have.
Valve Max - this option forces the R C I graph located at the top right of the screen to stay positive no matter what.
Water Reclaim - allows you to bulldoze over water and turn it into land.
Memory - This option is important. To enable all cheats you set on 'ON', put memory on 'SET', press B, and those cheats will become active. This cheat is also saved so you won't have to input the code over again. To clear the debug mode option, put memory on 'CLS', press B, and the debug menu and all cheats have been disabled. To re-activate it, input the code again.