Demo

Left 4 Dead 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 an update released November 10, 2008.

The demo was released to the public for the Xbox 360 and PC on November 11, 2008.

Availability
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 November 6th, 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 November 10th update.

Yahn Bernier from Valve Software said this about the disabling of 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.

Left 4 Dead 2 Demo
The Left 4 Dead 2 demo became available to everyone with Xbox Live Gold on November 3, 2009. People who pre-ordered however, were able to play it a week earlier, Due to delays the PC demo was released October 28 at 7 o'clock (PC users were able to pre-load it on October 24 through Steam). Users with Silver Xbox Live accounts were able to play the demo on November 10. Unlike the first demo, the demo is not disabled after the release of the actual game, although it cannot be played by those who purchased it.

Content
The Left 4 Dead 2 demo includes the first two levels of the The Parish campaign, similar to the original Left 4 Dead demo that included the first two levels of the No Mercy campaign, some new weapons and all of the new Special Infected (The Spitter, The Charger and The Jockey). This includes 4 melee weapons: Machete, Guitar, Nightstick and Frying Pan.

Trivia

 * There are many items that do not appear in the demo. There are only four melee weapons available in the demo; Axes, Crowbars, Chainsaw, Katanas, Baseball Bats, and Cricket Bats are not present. Grenade Launchers, laser sights, and Incendiary/Explosive ammunition are also not present, although can be accessed through console commands.
 * You can play as infected through downloading "hacking programs"

Glitches

 * There is a glitch that allows you to get to the top of the fences at the start of the campaign, you jump out towards the water but not to far off of the concrete barriers and you should catch onto the chain-link fence with barbwire on top, if a survivor comes along fast enough to help you up, you'll be at the top and relatively safer from Common Infected and Special Infected, you will still be vulnerable to Smokers, Jockeys, Hunters, and Boomer Bile if he gets close enough though.
 * There is a rain tent right after you get out of the the park that has health under it, if you jump on the generator near it onto the rain tent and look to your right, right above the alley you need to go through, if you jump on the spikes the effect is about the same as the glitch above, but Common Infected and The Witch will stand under you and spin in circles as if you are too high.