Type in the following code during gameplay:
Demo or Full Version:

Hold down SHIFT and type FUNDS-Gives you LOTS of money

 

If you press shift or CAPS LOCK [it doesn't matter] and type in fund your funds will increase $10,000, but watch out because if you go higher than $79,999 a major I mean MAJOR earthquake will occur! Then if you press it again the same will happen at 70k, 60k, 50k etc.

A tip is to set the citizen's taxes to 0%.  Then just before tax
collection (December or November if playing at fast speed) quickly set the taxes up to 20%.  Then after you collect taxes, put it back down to 0%.
This way the citizens think they are not paying any taxes.

Want $2 billion?  Try this:
go to the dos prompt.
go to your simcity folder and go to your city folder inside of your simcity folder.
type: DEBUG   filename.CTY
you'll get to a prompt like this : Debug -            
type at the prompt: E D24 7F and press enter
type this too: E D25 FF and press enter
type this to write it to your city :W
type this to exit: Q
       
This will affect the year of your city.  No disasters will occur.

mega cheat
this cheat will give you 500,000 and make all rewards available
This will only work for the dos version only

first prepare the cheat by...
1, pausing your city
2, selecting the centering tool.

1, press CTRL-F3 on the keyboard
2, click on the ROTATE COUNTER-CLOCKWISE (left button)
3, Click on the RESIZE BUTTON (located on the lower right hand corner of
the city edit window)
4, click on the SMALL STATUS WINDOW (the one that shows the weather,
price, and demand messages
5, click on the POPULATION ICON ( located on the toolbar(
6, click anywhere inside the CITY EDIT WINDOW
7, pull down the DISASTERS MENU, and then move the cursor on the between riots and no disaters. Release the mouse botton over the line. Note when the cursor in over the line, the dark highlight will disappear. this is what you should do.
8, after step 7 is finished you should get an extra 500,000 (see note
below), all rewards should be available

NOTE: you can only gain an extra 500,000 when you current funds are
below 500,000. If they are above , the cheat will not work. You may
repeat this as many times as you want

This cheat will give you lots of money.
Make sure you back up your city before doing this code.
works on doe versions only.

1, Start a new city.
2, pause the game
3, save your city and quit
4, at the dos prompt type YOURCITY.SC2 and pres enter(example maxis.sc2)
5, type E 127 7F and press enter
6, type W and press enter
7, type Q and press enter
8, now restart your city