You can gain access to one of two safehouses in Fallout 3 by completing the 'Power of the Atom' quest. Depending on the route you take to complete the quest you will either be given the key to a house in Megaton by Lucas Simms or the key to a penthouse suite in Tenpenny Tower by Alistair Tenpenny. You can then use the safehouse to store items, bobbleheads and to rest and heal your character. You can also "rent" (permanently) a room in the Rivet City Weatherly Hotel.