Demo

The Left 4 Dead demo was initially released on 6 November, 2008. It contains the first two chapters of the No Mercy campaign, playable in the Survivors' co-op mode, both locally and online. Initially, through a series of console commands, players could play as the Infected. This ability was removed in a 10 November, 2008 update.

The demo was released to the public for Xbox and PC on 11 November, 2008.

Preorders
The demo for Left 4 Dead was originally intended for those who pre-ordered the game through the Steam network or through GameStop or EB Games. Those who ordered through Steam received the demo automatically and instantly, however, people who used GameStop needed a demo code e-mailed to them. Many did not get theirs on time, adding to the already negative outlook on GameStop. Additionally, GameStop.com's system of distributing codes contained a glitch that allowed users to enter a random set of numbers or even leave the transaction code blank and still receive a code, allowing many to get the demo without pre-ordering.

The demo was disabled when Left 4 Dead was released.

Exploits
When released to select users on 6 November, 2008, the Left 4 Dead PC demo was open to many console-based exploits, including the sv_cheats command that allowed a user to essentially play the role of the AI Director by spawning infected by using the z_spawn commands. It was eventually discovered that with a console command, players could play as Infected, a feature that was originally planned only for the Versus mode in the full game. Also discovered was the PC split-screen feature that allowed 2 players to interact on the same computer with the help of an additional controller. Both of these exploits were disabled in the 10 November update.

Yahn Bernier from Valve software, said this about the disabling patches:

"Splitscreen was always something we only wanted to officially support on the x360 due to the controller issues. However, we are planning on making some of the functionality work on the PC as an "unsupported" bonus feature. We turned it off for the demo because we didn't want to have to deal with any additional bug reports etc. about it interfering with the demo experience, but the current plan is to re-enable it after we launch the full product."

Some players also used Valve's system of using similar map files to import maps of other Source games for use with Left 4 Dead's gameplay. The popular de_dust from Valve's Counter-Strike was among the first maps to be used in this way.