This is a bit of a tutorial on how to have (even more) fun with L4D!  I want to give credit to Barracuda from the Steam Forums for putting together some of these commands, most of this is his work not mine.

Ok first of all you will need to have the console enabled and this will require you to have a standard english keyboard as apparently some folks are having problems opening the developer console on non-english keyboards. 

Developer Console

1. Open the Options Menu.
2. From there, open the Keyboard/Mouse menu.
3. Adjust the "Allow Developer Console" setting to "Enabled"
4. Click "Done".
5. At any time, including menus as well as while playing, you may now hit '~' to open the developer console.  Don't worry too much about messing anything up, commands are not persistent between executions and if you stick to the commands you know and use reasonable values there are no problems.

Creating Hordes of Zombies

1. This, of course, requires the developer console to be enabled (see above).
2. In the console enter the following commands, note that values in brackets '[]' are the suggested range and values in parentheses '()' are the default values:
   • sv_cheats 1
   • director_force_panic_event
   • director_panic_forever [1] (0)
     
   • z_background_limit [20 to 300] (20)
     
   • z_common_limit [30 to 300] (30)
     
   • z_mega_mob_size [50 to 90] (50)
     
   • z_mob_spawn_max_size [30 to 60] (30)
   • z_mob_spawn_min_size [10 to 25] (10)
     
   • z_witch_speed [300 to 500] (300)
     
   • z_witch_speed_inured [200 to 300] (200)
     
   • OPTIONAL: god [1] (0)
3. An example of how to enter a command with a numeric argument is as follows "z_background_limit 220"
4. I heavily advise you start out in the middle of these ranges and work your way up as you need to, some systems won't be able to handle the high end of these ranges and some very nice systems will probably be able to go beyond them a bit.
5. Enjoy!

Playing as Zombies in the Demo

1. Some of you may have heard about this but Valve patched the game and the "feature" has been disabled.  It was a bit complicated anyways in all honesty, but very fun.

Also, if you're looking to play around you can use the "give" command along with shotgun, rifle, huntingrifle, pipebomb, autoshotgun, etc... to give yourself a different weapon or simply refill your ammo.  In addition you can fly around the level (and through walls) with the "noclip" [0 or 1] (0) command.

PS - Feel free to add any other ideas or things you guys have done to have fun in the game.

Aw, they patched the zombie feature. It definetly wasn't complete but having a friend play the tank was definitely a lot of fun. Anyways, I feel like the Demo alone could win GOTY, can't wait for the full thing.

Cheats are very fun in this game hehe.