Unlockable Levels:
Beat the six game scenarios and have at least one city with over 100,000 citizens living in it saved to the game - Las Vegas Scenario

Beat the Las Vegas scenario - Freeland

999 extra city maps:
There is normally only 999 city maps, but if for any strange reason you need 999 more, here's a way to unlock 999 more maps. Start a new city, and choose any map number, then start the game. Now select the ''Go to Menu'' Icon located at the top of the screen, and choose Start New City without saving. After a short period of time, the map will change, but the number wont.

$999,999:
At any time while playing a city, spend all of your money. When the tax screan apears at the end of December hold down L. Select ''Go With Figures'' and then go back to the tax screan. Turn all of the dues to 100% and exit still holding L. When you release L your money will be at $999,999.

Get away with not paying funds:
As far as I know, Auto-Budget must be off to use this code. When the Budget screen comes up at the end of the year, set all funds to 0%. Now select Go With Figures. Immediately save the game, then reload it. The game should continue as if you set all funds to 100%, but you weren't charged for them on the budget.