Settlers 1 -Serf City
Type in PASSIVE as password and you can play all the levels...

Settlers 2 -Veni, Vidi, Vici
To cheat in Settlers 2 just type "THUNDER". This will do it possible to build anything. Then if you want to see the whole map, just press F7. For more speed just hold alt + 1-6. If you got the Settlers 2 version 1.51 of the game the cheat code is "WINTER" instead of "THUNDER.

Settlers 3 -On land, On sea, On-line
Settlers 3 levelskipping (Trainer)