Press Shift and the ~ (tilde) key to bring up the console and type:

JUGGERNAUT
God mode

GOLDBLUM
Fly mode

WRAITH
No clipping mode

POSITIONS
Show your position (x,y,z)

A_MOVEME X,Y,Z
Move to the given co-ordinates

DIR MAPS
List available maps

MAP X
Warp to map x (x=3DLevel name)

SPAWN X
Create an item or creature

ODE TO JACK
Kill all creatures on level