Weapon mod

Weapon mods, while they have appeared in one way or another in Fallout, Fallout 2, and Fallout 3, they appear as an active feature for the first time in Fallout: New Vegas due to their popularity as a 3rd party mod in Fallout 3.

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.

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