Fallout: New Vegas/Factions

Factions

 * New California Republic (NCR)
 * Powder Gangers
 * Great Khans
 * Brotherhood of Steel

Lesser Factions (No Quests)

 * Fiends
 * Jackal Gang

Encountering New Factions
When you first encounter a member of a new faction, they will not be hostile to you; only because you didn't kill any their friends or pick-pocketed or stolen them.