Upgrades are a scrapped feature in Left 4 Dead. These upgrades are built into the game, but can only be activated by using the command "Survivor_upgrades 1", which can only be used by those running a dedicated server in the PC version. The upgrades are given to players at random at the start of a map. It is unknown what exactly determines what upgrades are obtained.
Server mods have expanded the system so that the player gets upgrades as rewards for certain acts, such as killing a Tank or surviving a Crescendo Event. Some servers have also added new upgrades, such as incendiary ammo.
Interestingly, upgrades have never been mentioned nor seen in any pre-release materials. This suggests that they were either developed in secret, then scrapped or were scrapped early in development and were kept in the code because it was too difficult to get rid of them.
A line in the file intructor_lessons.txt ("// 70 pick up a primary weapon (or upgrade)") hints that upgrades could be picked up in levels. However, in the final game, the only way to obtain them is for them to randomly appear at the start of a map.
This item reduces the amount of damage you take.
Protects you from a Boomer's bile once.
Allows a person to pick him/herself up from a ledge once automatically.
Allows the player to revive him/herself once automatically.
Prevents a player from being affected by the "slime" image when a player is hit by the Boomer's bile.
Increases current health to 150.
Adds a laser light that comes from the flashlight. This disables the crosshair.
Reduces weapon recoil.
High capacity magazineEdit
Gives the player a larger magazine with each of their weapons, though the exact percentage increase is unknown.
Hollow point ammoEdit
Increases the amount of damage weapons do.
Allows a player to revive a downed player faster once.
Allows a player to run faster when wounded.
Allows the user to reload faster.
This one is only referenced in "game_sounds_weapons.txt". Based on its name, it seems that it would behave similar to a flashbang grenade.
Listings for "skills" are found deep in the game's files. It is unknown how these would work, as no working code exists for them. They might be similar to the "classes" in other Valve games, such as Team Fortress 2. A server command theoretically to theoretically activate these skills, "survivor_skills 1" exists, but using it causes all human players to lose the ability to control their characters and they obtain no "skills."
A small upgrade system is implemented in Left 4 Dead 2, including the Laser Sight originally meant for Left 4 Dead and incorporating the new Explosive Ammunition and Incendiary Ammunition. Early footage- http://www.youtube.com/watch?v=a3Uuxn0pRKE -shows a player picking up a Laser Sight.