The cheat boolprop TestingCheatsEnabled true (also known as just 'the boolprop cheat') is the most powerful cheat in the game. This cheat was most likely used by Maxis to test the game (hence the name) and really wasn't originally meant to be known.
However even though this is 'just' a cheat, it is not to be taken lightly; It can alter many parts of the game and for several people it caused problems. With enough caution however, it can be a really fun and usefull cheat.
After you're done using this cheat, it is very recommended to stop this cheat from working by using boolprop TestingCheatsEnabled false
Below you can find several uses of this cheat. Every link goes to a seperate tutorial and so some parts will be repeated.
Note that there are an enormous amount of uses of this cheat; too many to cover here, but the most usefull and interesting ones can be found in this list.