Hold Shift and press ~ (tilda) to bring down a menu then type the
following cheats:

BIGCHEATER - God Mode
or
JUGGERNAUT 
     (Oddly,  in  some  games  it  is  BIGCHEATER, and in others it is
JUGGERNAUT. Try both.)

NOCLIP - No Clipping mode (walk through walls)
or
WRAITH
(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.