Start game as usual, open up the console with ~ key and enter following
commands as desired. 0 instead of 1 disables the cheat and vice versa.
Entering the command without any values will show the value, it is
currently set to. Disable the console (press ~ again) to continue
playing the game.

god 1 - ENABLE GODMODE (default 0)
disable_ai 1 - ENEMIES WONT ATTACK (default 0)
noclip 1 - WALK THROUGH WALLS (default 0)
gravity 0 - NO GRAVITY (default 1)
phizik 0 - ??? (default 1)
speed 300 - RUN FASTER (default 125)
debug 1 - ENABLE DEBUG MENU (fps/coordinates/etc.)
drawpersons 0 - ??? (default 1)
pers_move_check 0 - ??? (default 1)
pers_move 1 - ??? (default 1)
shadows 1 - ENABLE SHADOWS (default x)
smooth_shadows 1 - ENABLE SMOOTH SHADOWS (default x)
wave_trace 0 - ??? (default 1)
addtool 1 - ??? (default 0)
newtrace 0 - ??? (default 1)
movie 1 - ??? (default 0)
bump 1 - ENABLE BUMP MAPPING? (default x)
portals 0 - ??? (default 1)
wire 1 - SHOW WIREFRAME (default 0)
lod 0 - LEVEL OF DETAIL? (default x)
hudenable 0 - DISABLE HUD (default 1)
fps - SHOW FPS (default 0)
vshaders 1 - VERTEX SHADERS (default x)
persons 1 - ??? (default 0)
mover 1 - ??? (default 0)
ragdoll 1 - ??? (default 0)
contacts 1 - ??? (default 0)
enabled 1 - ??? (default 0)
nakmodel 1 - ??? (default 0)
trimesh 1 - ??? (default 0)
selfshade 1 - ??? (default x)
person_bump_dist 1- ??? (default x)
obj_bump_dist 1 - ??? (default x)
obj_bump_dist 1 - ??? (default x)
drawstatic 1 - ??? (default x)
drawsmoke 1 - ??? (default x)
checksmoke 1 - ??? (default x)
calcsmoke 1 - ??? (default x)
objshadows 1 - ??? (default x)
ps 20 - ENABLE PIXEL SHADER 2.0? (default x)