Press (~,Ctrl, and Alt) to bring up the game console during play and enter the following codes to activate them. Make sure you press "Enter" after you type each code in!

give item_health_mega -> +100 health up to 200
g_showplayershadow # -> 1 shows shadow, 0 removes it
com_showfps # -> 1 to show frame rate on screen constantly, 0 to turn it off
pm_thirdperson # -> 1 turns third-person mode on, 0 turns it off
give all -> All weapons with full ammo, health and armor
com_allowconsole 1 -> Allows console opening and closing with just ~
editor -> Close the game and open the map editor
kill -> commit suicide
testlight -> Creates a new light source where you're pointing
benchmark -> game benchmark
give weapon_dmg -> get a dark matter gun plus some ammo
give weapon_grenadelauncher -> get a grenade launcher plus some ammo
give weapon_hyperblaster -> get a hyperblaster plus some ammo
give weapon_lightninggun -> get a lightning gun plus some ammo
give weapon_machinegun -> get a machinegun plus some ammo
give weapon_nailgun -> get a nailgun plus some ammo
give weapon_railgun -> get a railgun plus some ammo
give weapon_rocketlauncher -> get a rocket launcher plus some ammo
give weapon_shotgun -> get a shotgun plus some ammo
give weapons -> get all weapons without any ammo
give ammo -> give ammo
give all -> weapons gives all weapons
gfxinfo -> graphics card information
pm_noclipspeed # -> How fast you move in noclip mode
pm_crouchspeed # -> How fast you move when crouched
pm_speed # -> How fast you move when running
pm_jumpheight # -> How high you can jump
quit -> immediately exit from game
give armor -> max armor
give health -> max health
killmonsters -> remove all monsters and NPCs from current level
poplight -> Removes a "testlight"-created light source
screenshot X Y -> replace X and Y with 4:3 screen resolutions, like 800 600 or 1024 768
avidemo -> save a demo of your playing to an AVI movie file
g_knockback # -> Set to 0 to keep monster attacks from knocking you around
g_stoptime # -> Set to 1 to stop time (can only move around) and 0 to restart time
spawn char_marine -> summons a marine who'll help fight
spawn char_marine_medic -> summons a medic who can heal you
spawn char_marine_tech_-> armed summons a tech who can fight and restore your armor
noclip -> toggle ability to fly and move through solid objects
GOD -> Toggle GOD Mode
notarget -> toggle invisibility