Power to the People
Power to the People These rare and wondrous machines will grant a one-time upgrade to any of your weapons (excluding the Wrench and Research Camera). There are two upgrades available for each of the six primary weapons, which means there are twelve upgrades in all. Since Fontaine Futuristics has installed a dozen Power to the People stations in Rapture, you should be able to snag every upgrade available if you manage to find every station. For easy reference, I've listed all available upgrades below:

Chemical Thrower Consumption Rate Chemical Thrower Consumption Rate
A tapered nozzle reduces the ammunition consumption rate of the Chemical Thrower. If you use the Chemical Thrower often, grab this upgrade after taking the range upgrade below.

Chemical Thrower Range Chemical Thrower Range
A pressure-calibrated hose increases the range of the Chemical Thrower. A must-have upgrade if you spend a lot of time wielding the Chemical Thrower.

Crossbow Breakage Chance Crossbow Breakage Chance
An automatic tension adjuster greatly reduces the chance of bolts breaking on impact. Leave this upgrade for late in the game unless you like sniping opponents from a distance.

Crossbow Damage Increase Crossbow Damage Increase
A high-tensile bow increases the damage done by crossbow bolts. Take this upgrade as soon as it becomes available.

Grenade Launcher Damage Increase Grenade Launcher Damage Increase
Incendiary injectors increase the damage of the Grenade Launcher. All of the above ammunition really packs a punch once you have this upgrade.

Grenade Launcher Damage Immunity Grenade Launcher Damage Immunity
Autogenerated shape charges make you immune to the damage from your own grenades. An excellent upgrade, especially if you spend most of your encounters with Rapture's denizens up close and personal.

Machine Gun Damage Increase Machine Gun Damage Increase
An acceleration framework increases the damage of the Machine Gun. I wouldn't recommend taking either of the Machine Gun's upgrades until you've taken most of the others.

Machine Gun Kickback Reduction Machine Gun Kickback Reduction
A stabilization cylinder reduces the kickback of the machine gun. I wouldn't recommend taking either of the Machine Gun's upgrades until you've taken most of the others. This should be your first choice of the two, though.

Pistol Clip Size Pistol Clip Size
An auto-loading clip effectively quadruples the clip size of the pistol. And that it does - from 6 to 24 bullets. I have no idea why a revolver has a clip, though.

Pistol Damage Increase Pistol Damage Increase
An ammunition accelerator increases the damage done by the pistol. Don't bother with this upgrade until there's nothing better to take.

Shotgun Rate of Fire Shotgun Rate of Fire
A steel gear accelerator increases the shotgun's rate of fire. An amazing upgrade that should be taken as soon as possible.

Shotgun Damage Increase Shotgun Damage Increase
A gas-assisted firing mechanism increases the shotgun's damage. Definitely one of the best upgrade choices in the game.