I was playing this game just fine until I got to level 18 where you need to propel yourself high enough to reach upper platforms. ..well I couldn't do it so I created a moon-jump.
The game really doesn't need any cheats but it does tend to give you a headache popping in and out of portals at diffferent angles so this may prove useful too others.
http://depositfiles.com/files/2089713]Portal
Levitate
Infinite Health