Upgrade

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.

Obtaining
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.

Kevlar Vest
String: #L4D_Upgrade_kevlar_vest_description

This item reduces the amount of damage you take.

Prevent it
String: #L4D_Upgrade_prevent_it_description

Protects you from a Boomer's bile once.

Ledge Save
String: #L4D_Upgrade_ledge_save_description

Allows a person to automatically pick him/herself up from a ledge once.

Second Wind
String: #L4D_Upgrade_revive_self_description

Allows the player to automatically revive him/herself once.

Goggles
String: #L4D_Upgrade_goggles_description

Prevents a player from being affected by the "slime" image when a player is hit by the Boomer's bile.

Hot Meal
String: #L4D_Upgrade_hot_meal_description

Increases current health to 150.

Laser Sight
String: #L4D_Upgrade_laser_sight_description

Adds a laser light that comes from the flashlight. This disables the crosshair.

Combat Sling
String: #L4D_Upgrade_combat_sling_description

Reduces weapon recoil.

High capacity magazine
String: #L4D_Upgrade_high_cap_mag_description

Gives the player a larger magazine with each of their weapons, but the exact percentage increase is unknown.

Hollow point ammo
String: #L4D_Upgrade_hollow_ammo_description

Increases the amount of damage weapons do.

Knife
String: #L4D_Upgrade_knife_description

Allows the user to either escape from a Smoker's tongue or get out of a Hunter's pounce once. The knife must be activated by the user.

Smelling Salt
String: #L4D_Upgrade_smelling_salt_description

Allows a player to revive a downed player faster once.

Ointment
String: #L4D_Upgrade_ointment_description

Allows a player to run faster when wounded.

Reloader
String: #L4D_Upgrade_reloader_description

Allows the user to reload faster.

Flashbulb
String: #L4D_Upgrade_flashbulb_description

Unknown.

Blinding Flash
This one is only referenced in "game_sounds_weapons.txt". Based on it's name, it seems that it would behave similar to a flashbang grenade.

Skills
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 to theoretically activate these skills, "survivor_skills 1", exists, but using it causes all human players to loose the ability to control their characters and they obtain no "skills".

55. aPackrat

56. aCommando

57. aOpenfire

58. aDefender_0

59. aGuide

60. aKnuckles

61. aEngineer

62. aSprinter

63. aGroupie

64. aClimber

65. aSoldier

66. aDemolitions

Return
It seems that some upgrades might be making a return in Left 4 Dead 2. Pre-release footage shows that the player can obtain an optional laser sight that behaves exactly like the one in L4D. This can be seen in this video - http://www.youtube.com/watch?v=a3Uuxn0pRKE - where some of the survivors have laser sights.