Fallout: New Vegas Factions

Major Factions
The following factions are the 3 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.
 * New California Republic
 * Caesar's Legion
 * Robert House
 * Yes Man

Minor Factions
The following factions can be recruited as allies (of you and the major faction you ally with, if any) in the climactic battle.
 * 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 3 major factions. All three of these factions run casinos on The Strip.
 * Chairmen
 * Omerta
 * White Glove Society

Other Factions

 * Merchant companies
 * Crimson Caravan
 * Van Graffs
 * Gun Runners
 * Raider Gangs
 * Fiends
 * Jackals
 * Vipers
 * Scorpions
 * Towns
 * Goodsprings
 * Freeside
 * The Strip
 * Novac
 * Others
 * Bright Brotherhood
 * State of Utobitha
 * Kings
 * Powder Gangers
 * Ghost People

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.
 * POSSIBLE FIX: Find out which faction's armor is causing hostility from a group. Put it back on, and then make sure the Pip-Boy says that you are no longer disguised as a member of that faction when you unequip the armor.
 * If this doesn't work (it hasn't in my games), you'll have to use the console command "setreputation factionID x y". But the best bet is, NEVER wear faction clothing/armor unless you actually want the permanent hostility of that faction's enemies.
 * 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, 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.)