Hold Shift and press ~ (tilda) to bring down a menu then type the following cheats:
Take No Prisoners
BIGCHEATER - God Mode
(Oddly, in some games it is BIGCHEATER, and in others it is JUGGERNAUT. Try both.)
NOCLIP - No Clipping mode (walk through walls)
(Same as God mode: sometimes it is one or the other)
FLY- Flying mode
Usually BIGCHEATER, NOCLIP, and FLY are together.
Usually JUGGERNAUT and WRAITH are together.
Thanks to Revolution readers Charlie Dykstra, Mike Geschke, Justin, and Tim Bo-Lassen!