Fallout: New Vegas weapon mods

Overview
Several of New Vegas's weapons can be modified with mods.

There are multiple weapon mods for some weapons, and a maximum of three weapon mods can be attached to any one weapon. Once any weapon mod is attached it is permanent, however there are multiple of each mod in the game, 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 makes it harder for close combat.

Mods are attached by having both the weapon and the mod in your inventory, then going to the weapons page on the Items tab within your PipBoy, highlighting the weapon you want to mod, and looking to the right of the info pane to see what button or key to use to open up the mod screen for that weapon. After you have selected a mod, the weapon name will have a + after it to show it has been improved. Modding a weapon also increases its value.

Unique weapons cannot be equipped with weapon mods. While unique guns usually have better base stats, Obsidian Entertainment introduced this inherent drawback to balance out unique and normal weapons and make the choice less of an obvious decision and more of a preference.

Vendors
There are currently 11 vendors who offer weapon mods (inventory of mods changes with stock rotation) in the Mojave Desert:

Do note that killing the vendor doesn't allow you to pick up the weapon mods from their stashes, or bodies.
 * Alexander, 188 Trading Post (Need at least 40 guns skill to pass challenge)
 * Blake, Crimson Caravan Camp
 * Chet, Goodsprings
 * Cliff Briscoe, Novac
 * Dale Barton, The Fort
 * Gloria Van Graff, Silver Rush, Freeside
 * Knight Torres, Hidden Valley
 * Michelle, 188 Trading Post
 * Mick (Fallout: New Vegas), Mick and Ralph's, Freeside
 * Old Lady Gibson, Gibson Scrap Yard
 * Vendortron, Gun Runners