Weapon mod

, while they have appeared in one way or another in Fallout, Fallout 2, and Fallout 3, appear as an active feature for the first time in Fallout: New Vegas.

Fallout
Only Smitty can perform modifications in Fallout, and the only weapon he can upgrade is the Winchester P94 into a Turbo Plasma Rifle.

Fallout 2
In Fallout 2, you can pay a mechanic to upgrade certain weapons for you.

List of available weapons

 * Desert Eagle .44 → Desert Eagle (Exp. Mag.)
 * .44 Magnum Revolver → .44 Magnum (Speed Load)
 * AK-112 Assault Rifle → Assault Rifle (Exp. Mag.)
 * Colt Rangemaster Hunting Rifle → Scoped Hunting Rifle
 * FN FAL → FN FAL (Night Sight)
 * Wattz 1000 Laser Pistol → Magneto-Laser Pistol
 * Wattz 2000 Laser Rifle → Laser Rifle (Ext. Cap.)
 * Glock 86 Plasma Pistol → Plasma Pistol (Ext. Cap.)
 * Winchester P94 Plasma Rifle → Turbo Plasma Rifle
 * Flamer → Improved Flamer
 * Big Frigger Power Fist → Mega Power Fist
 * Cattle Prod → Super Cattle Prod

List of merchants

 * Valerie, in Vault City
 * Skeeter, in Gecko
 * Eldridge, in New Reno
 * Algernon, in the basement of New Reno Arms. Works for free.

Fallout 3
The only upgrades seen in Fallout 3 are scopes, the occasional use of silencers, and the use of extended magazines. Other differences between unique weapons do not technically count as weapon mods, seeing as there is only one of each unique weapon. There is no way to get any weapon upgraded.

Fallout: New Vegas
Any of New Vegas's weapons can be modified with several mods.

There are multiple weapon mods for every weapon, and a maximum of three weapon mods can be attached to any one weapon. Once any weapon mod is attached it is permanent, so be wise in what you want to attach. For example, while an extended magazine is almost always beneficial, a scope has pros and cons. It may increase maximum range, but it may also worsen your ability to aim closer to yourself, making it harder for close combat.

List of merchants
Name, location, type of upgrades available

Chet, in Goodsprings, Guns

Great Khan Armorer, in Red Rock Canyon, Guns

Blake, with the Crimson Caravan Company, Energy Weapons and Guns

Vendortron, with the Gun Runners, All types

Gloria Van Graff, in Silver Rush, Energy Weapons

Dale Barton, in The Fort, Energy Weapons and Guns

Michelle, in 188 Trading Post, Guns

Knight Torres, in Hidden Valley, Explosives and Guns

Old Lady Gibson, at the Gibson Scrap Yard, Energy Weapons

Cliff Briscoe, at Novac, Energy Weapons and Guns

Appearances
appear in Fallout, Fallout 2, Fallout 3, and Fallout: New Vegas.