Left 4 Dead

This is a personal favorite. I have recently started copy/pasting from gameFAQs becouse it is alot easier. Please enjoy these cheats.

At the main menu, select "Options," then "Keyboard/Mouse." Enable "Allow Developers Console." During gameplay, press "~" to access the console. Type in "Sv_Cheats 1" and press "Enter" to enable the cheats. Then, enter any one of the following:

tongue_range <#> (Default 750)
Sets the maximum range of the smoker's tongue attack

z_witch_burn_time
Amount of time before a burning Witch dies from fire (does not affect damage caused by fire)

z_health
Change Zombie Health

z_speed
Change Zombie Speed

director_no_human_zombies
0/1 Disables/enables controllable Boss Infected on any map

director_panic_forever
0/1 Enables/Disables never ending panics.

director_force_panic
Forces a panic event

give ammo
Full Ammunition

give pistol
Get Pistol

impulse 101
Gives Ammo To Your Primary Weapon

give autoshotgun
Gives auto shotgun

give health
gives full health

give gascan
gives gascan

give hunting_rifle
gives hunting rifle

give molotov
gives molotov

give oxygentank
gives oxygen tank

give pain_pills
Gives pain pills

give pipe_bomb
gives pipebomb

give propanetank
gives propanetank

give rifle
Gives you an Mk 47 (machine gun)

give first_aid_kit
gives you first aid kit

z_spawn weapon_SMG
Gives you the SMG.

god
Invincibility

buddha 1
just like god but you receive damage(sometimes if you are the only survivor left you can't receive damage from infected)

director_no_mobs <1/0>
No mob rushes

firstperson
Play in first person (default); this cheat is only necessary if thirdperson mode is enabled

z_witch_damage <#>
Sets the amount of damage a Witch's attack does.

thirdperson_mayamode
Sets the camera to be fixed in place while in either third person or third person shoulder mode. Enter once to turn on and enter again to turn off.

thirdperson
Sets the camera to be in third person mode.

thirdpersonshoulder
Sets the camera to over-the-shoulder third person mode

z_common_limit
Sets the maximum amount of regular zombies

z_tank_health <#>
Sets the maximum health of the Tank

z_frustration_lifetime <#>
Sets the number of seconds until the player loses control of the Tank from not attacking survivors

sb_friendlyfire <1/0>
Sets whether or not bots (and only bots) are allowed to do friendly fire damage

director_stop
Shuts off all wanderers, mobs, specials, and bosses

z_spawn zombie
Spawns 1 Zombie

boom
Spawns a active pipebomb under you

fire
Spawns a lighted molotov right on the floor under you.

z_spawn <bossname>
spawns a special infected (infected names:hunter,boomer,smoker,witch,tank)

z_spawn mob s
pawns a zombie horde

z_spawn boomer
Spawns boomer at location of crosshair

z_spawn hunter
Spawns hunter at location of crosshair

z_spawn smoker
Spawns smoker at location of crosshair

z_spawn tank
Spawns tank at location of crosshair

noclip
Toggles No Clipping Mode

z_witch_allow_change_victim <1/0>
True/False setting that allows the witch to change targets instead or focus on the one survivor that alerted her first

sv_infinite_ammo 1
Unlimited Ammunition

retry
You can unlock achievements while using cheats (after entering sv_cheats 1, enter retry so that it will restart the game with the cheats on)
found on: http://www.gamefaqs.com/computer/doswin/code/937406.html