Fallout: New Vegas/Factions

Factions

 * New California Republic (NCR)
 * Powder Gangers
 * Great Khans
 * Cesares Legion

Lesser Factions

 * Fiends
 * Jackal Gang
 * Brotherhood of Steel

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, pick-pocketed or stolen them. Except the lesser factions, they'll attack you on sight.