During game play press '~' (tilde) key, Ctrl and Alt to bring up the game console to be able to enter the following codes to activate the corresponding effect.

Toggle GOD Mode:
GOD

Get ALL Weapons:
Give all weapons

Get Ammo:
Give ammo

Get Max Armor:
Give armor

Get Max Health:
Give health

ALL Weapons with Full Ammo, Health and Armor:
Give all

Get a Dark Matter Gun plus some Ammo:
Give weapon_dmg

Get a Grenade Launcher plus some ammo:
Give weapon_grenadelauncher

Get a Hyperblaster plus some Ammo:
Give weapon_hyperblaster

Get a Lightning Gun plus some Ammo:
Give weapon_lightninggun

Get a Machine Gun plus some Ammo:
Give weapon_machinegun

Get a Nailgun plus some Ammo:
Give weapon_nailgun

Get a Railgun plus some Aammo:
Give weapon_railgun

Get a Rocket Launcher plus some Ammo:
Give weapon_rocketlauncher

Get a Shotgun plus some Ammo:
Give weapon_shotgun

Get ALL weapons without any Ammo:
Give weapons

How Fast you move in No Clip Mode:
Pm_noclipspeed #

How Fast you move when Crouched:
Pm_crouchspeed #

How Fast you move when Running:
Pm_speed #

How High you can Jump:
Pm_jumpheight #

+100 Health up to 200:
Give item_health_mega

Shows Shadow:
G_showplayershadow # (1 Shows and 0 removes it)

Show Frame Rate on Screen Constantly:
Com_showfps # (1 to show and 0 to turn it off)

Third-Person Mode:
Pm_thirdperson # (1 to shows and 0 to turn off)

Allows console opening and closing with just '~' (tilde key):
Com_allowconsole 1

Close the Game and open the Map Editor:
Editor

Creates a New Light Source where you point:
Testlight

Game Benchmark:
Benchmark

Graphics Card Information:
Gfxinfo

Immediately Exit from Game:
Quit

Spawns a Mmarine who'll help Fight:
Spawn char_marine

Spawns a Medic who can Heal you:
Spawn char_marine_medic

Remove ALL Monsters and NPCs from current Level:
Killmonsters

Spawns a Tech who helps fight and can restore your Armor:
Spawn char_marine_tech_armed

Toggle Invisibility:
Notarget

Commit Suicide:
Kill