To activate cheat codes, press ~. This will bring up a console. Enter "iamtheone" to enable Cheat Mode. Then enter the codes below to activate the corresponding function:

fly: Enable flying
ghost: No clipping
god: God mode
fly: Enable flying
Disable fly: walk
Suicide: suicide
Invisibility: invisible 1
Disable Invisibility: invisible 0
1st person view: behindview 0
3rd person view: behindview 1
Slow motion: slomo
Disable slow motion: slomo 1