Please, only join if you have L4D.

Here's how to cheat:
1)Enable the developer console. Do this by pressing (While ingame) Esc>Options>Mouse/keyboard then turn developer's console on. From now on press ~ to open up your console.
2) Type (sv_cheats 1) without the parentheses into the console and you're good to go.
3) Basic zombie spawning:(z_spawn common)without the parentheses to spawn a common infected. (z_spawn mob) which is a horde. (z_spawn hunter) (z_spawn smoker) (z_spawn boomer) (z_spawn tank) (z_spawn witch)
4)Some other cheats i use include:
god 1 (Invincibility)
sv_infinite_ammo 1 (Infinite ammo, you can hold down shoot forever)
5) Giving stuff to yourself
Type give weapon to give yourself whatever you want.
Here is a full list of what you can give yourself:
Pistol
Pump_shotgun
Autoshotgun
Rifle
Hunting_rifle
Pipe_bomb
Pain_pills
Molotov
first_aid_kit
Gascan
oxygentank
propanetank
6) spawning items on the ground: you must point at the ground. Type
ent_create weapon_(weapon here)
For example
ent_create weapon_Autoshotgun
would spawn an autoshotgun where i'm pointing.
7) Misc. If you are hanging off a ledge or incapped (Not likely if you have god on) type give health to get yourself back on your feet. If a tank is endlessly smashing you in a corner and you can't get away and your teammates are busy, point at the tank *Very important to point right at him* and type ent_remove. This will remove the tank, but be careful because ent_remove can remove anything! If you accidently point at yourself you will remove your player and will be stuck in spectator for the rest of the game. ent_remove works best if pounced, because it's easy to point at the hunter. However, smokers are your worst enemy because you usually can't see them when they drag you. if you use ent_remove you will remove your first person view and can't shoot!
8) Type noclip to fly. It's not really flying, but you can go wherever you want, fly through stuff, and go really high or under the ground. Careful, though-smokers can still pull you and hunters can still pounce you if you're low enough.
9) A cheat i recently discovered:
nb_blind
Makes all infected blind. They will not go after you. However, if you walk within melee range, they will still attack you.
9) Check out this site:
http://left4dead.wikia.com/wiki/Complete_list_of_console_commands
It's a 99.99% complete list of console commands for l4d.
10) I heard there's a plugin out there that lets 12 survivors play at once. Try it out, maybe we can all play on 1 server. It'll be fun!