The way of opening the console is CTRL+ALT+TILDE (~).

Basic codes
"god" - the player becomes invincible, lasting for one level
com_allowconsole 1 - Allows console opening and closing with just ~
"undying" - the player stops taking damage at one health and zero armor.
"give ammoregen" - the player's ammo regenerates over time
"give doubler" - the player will do twice as much damage as normal, the friendlies do the same damage. it lasts for one level.
"give haste" - the player's speed is increased for thirty seconds
"give quad" - the player will do quadruple damage for thirty seconds
"give all" - the player receives all weapons, armor, health and ammo. Note: Your weapons will not be upgraded before you reach the level where you obtain the upgrade from your teammates
"give weapon #" - # stands for the weapon you want to spawn. Note: Not all weapons need to key in its full name to get it. (i.e. Dark Matter Gun = give weapon_dmg)
"give guard" - the player receives an additional 100 armor depending on level and situation
"Pm_speed #" - the player's speed is increased; based upon what number is entered, lasts for one level.
"Pm_jumpheight #" - the player's jump is increased; based upon what number is entered
"Pm_thirdperson 0 or 1" - the player can toggle between first-person perspective ("0") and third-person perspective ("1"). The third-person may be unplayable due to the lack of the front view and HUD to tell your status.
"noclip" - This code makes it so you can walk through walls, floors, etc., using this cheat too much may cause the game to crash.
"notarget" - Enemies will not notice you, unlike Doom 3 or its expansion pack, this cheat will work on any enemy. Friendlies still can see you. The enemies will only attack friendlies if this cheat is enabled, making the escort missions harder. Sometimes scripted event cannot be triggered if this cheat is in effect.

Here are some more codes that will spawn marines, entities and even monsters for you. The armed human allies will carry Machine Guns only.

"spawn char_marine" - spawns a marine ally
"spawn char_marine_tech" - spawns a marine technician; unarmed
"spawn char_marine_tech_armed" - spawns a marine technician; armed
"spawn char_marine_medic" - spawns a marine medic; unarmed
"spawn char_marine_medic_armed" - spawns a marine medic; armed
spawn char_marine_hyperblaster - spawns a marine with a hyperblaster
spawn char_marine_shotgun - spawns a marine with a shotgun
spawn vehicle_walker - spawns a driveable mech
spawn vehicle_gev - spawns a driveable tank
"spawn monster_strogg_marine" - spawns a Strogg marine
"spawn monster_strogg_marine_mgun" - spawns a Strogg marine carrying a machine gun
"spawn monster_strogg_marine_sgun" - spawns a Strogg marine carrying a shotgun
"spawn monster_grunt" - spawns a Strogg Grunt
"spawn monster_berserker" - spawns Strogg Berserker
"spawn monster_gunner" - spawns Strogg Tank
"spawn monster_network_guardian" - spawns Strogg Network Guardian
"spawn light" - spawns an entity to light the player's way