Left 4 Dead Cheats

Developers Console 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:

give ammo: Full Ammunition
give pistol: Get Pistol
z_spawn weapon_SMG: Gives you the SMG
god: Invincibility
noclip: Toggles No Clipping Mode
sv_infinite_ammo 1: Unlimited Ammunition

Crash Course DLC Achievements
Complete each achievement to receive the allotted gamerscore.

20 Car Pile-up (20): As a Tank hit 20 Survivors with a car in the Crash Course campaign.
Crash-proof (15): Survive the Crash Course Campaign.
Jumpin' Jack Smash (25): Pounce a Survivor for 25 points of damage in the Crash Course campaign.
Quick Power (25): Restart the generator within 30 seconds of it shutting off in the Crash Course campaign.
Slippery Pull (25): Smoker pull a bile-covered Survivor until you hold him during Crash Course.
Smash Hit (25): Win a Versus campaign of Crash Course.
Tank Stumble (20): Stun a Tank with an explosion in the Crash Course campaign.
The Littlest Genocide (25): Kill 5,359 Infected in the Crash Course campaign.
Truck Stop (35): Your team wipes all Survivors after the escape vehicle has opened in Crash Course.
Wipefest (35): Your team incapacitates three Survivors within five seconds in the Crash Course campaign.