I've played Fallout 3 at my friends house, the storyline is very easy to complete in a couple hours and I've seen people do it with a Level 3 character, but the main part of it is exploring the Capital Wasteland (Washington D.C.)
There's some cool stuff to be found in there
*cough* National Guard Armory *cough*
But the game is very good, although it is Oblivion with guns.
There are also Downloadable Content that I believe is free if you have the game released by Bethesda, called Point Lookout, The Pitt, New Vegas, and Operation Anchorage.