Fallout: New Vegas Factions

Major Factions
The following factions are the four major players in New Vegas. During the course of the game, you will eventually have the option to side with one of them, or go it alone, to the expense of your relations with the other major factions.
 * Yes Man
 * New California Republic
 * Caesar's Legion
 * Robert House

Minor Factions
The following factions can be recruited as allies (of you and the major faction you ally with, if any.)
 * Boomers
 * Brotherhood of Steel
 * Followers of the Apocalypse
 * Great Khans
 * Enclave Remnants

The Strip Factions
In addition to the Brotherhood of Steel and the Great Khans, the following are parties you can choose to destroy or spare if you choose to go it alone instead of siding with one of the three major factions. All three of these factions run casinos on The Strip.
 * Chairmen
 * Omertas
 * White Glove Society

Other Factions

 * Merchant companies
 * Crimson Caravan
 * Van Graffs
 * Gun Runners
 * Happy Trails Caravan Company
 * Raider Gangs
 * Fiends
 * Jackals
 * Vipers
 * Scorpions
 * Towns
 * Goodsprings
 * Freeside
 * The Strip
 * Novac
 * Jacobstown
 * Primm
 * Others
 * Bright Brotherhood
 * State of Utobitha
 * Kings
 * Powder Gangers
 * Ghost People
 * Westside Militia
 * Sorrows
 * Dead Horses
 * White Legs

Bugs

 * Sometimes equipping ANY faction clothing/armor, no matter how briefly, PERMANENTLY sets you as a member of that faction in the eyes of all enemies of that faction. This happens even if you put on neutral duds before exiting Pip-Boy and no NPC ever sees you wearing enemy faction attire.  For example, while looting bodies out the wasteland you accidentally put on Powder Ganger armor but immediately take it off.  When you return to Primm, all residents will be hostile anyway.  If you ever put on Legion gear, the NCR will be hostile forever after, etc.
 * Trading faction armor you are wearing to a companion will result in an interface bug, as you are still in the trading screen when the "no longer wearing faction armor" comment pops up. (Furthermore, if the faction armor cannot be worn by the companion, as is the case for most of them (Veronica can wear power armor), they will drop the armor, anyway.)  This dialog can be impossible to select OK on.  Pressing start and hitting Continue appears to fix this.  (Unconfirmed on other platforms.)
 * If you are accepted, idolized,etc, by a faction and you wear their armor, it is possible that your reputation with that faction will drop to neutral. Even if you take the armor off, you'll still be neutral.It is possible for it to work the opposite way, if you're vilified, hated, etc, the same thing can happen. The only way to boost your reputation with that faction is to do quests for them.(Sometimes when you drop the armour your reputations will go back to normal.)