Full list of all 72 Fallout 3 achievements worth 1,550 gamerscore. It takes around 60-80 hours to unlock all of the achievements in the base game on Xbox 360.
The base game contains 50 achievements worth 1,000 Gamerscore, and there are 5 DLC packs containing 22 achievements worth 550 Gamerscore.
Completed "Following in His Footsteps"
Completed "Picking up the Trail"
Completed "Finding the Garden of Eden"
Completed "The American Dream"
Completed "Take it Back!"
Completed "The Superhuman Gambit"
Completed "Head of State"
Completed "Oasis"
Completed "Tenpenny Tower"
Completed "Strictly Business"
Completed "You Gotta Shoot 'Em in the Head"
Completed "Trouble on the Homefront"
Completed "Agatha's Song"
Reached Level 20 with Bad Karma
Won 50 Speech Challenges
Hacked 50 terminals
Kill all the Super Mutant Behemoths
Collected 10 Vault-Tec Bobbleheads
Collected 20 Vault-Tec Bobbleheads
Reached Level 20 with Neutral Karma
Reached Level 14 with Neutral Karma
Reached Level 14 with Bad Karma
Reached Level 8 with Neutral Karma
Reached Level 8 with Bad Karma
Completed "The Waters of Life"
Completed "The Nuka-Cola Challenge"
Discovered 100 locations
Made one of every custom weapon
Killed 300 people
Completed "Reilly's Rangers"
Reached Level 20 with Good Karma
Completed "Rescue from Paradise"
Placed a grenade or mine while pickpocketing
Completed "Tranquility Lane"
Completed "Scientific Pursuits"
Completed "Stealing Independence"
Completed "Galaxy News Radio"
Killed 300 creatures
Reached Level 14 with Good Karma
Completed "Big Trouble in Big Town"
Picked 50 locks
Completed "Blood Ties"
Completed "The Wasteland Survival Guide"
Reached Level 8 with Good Karma
Completed "The Replicated Man"
Completed "Those!"
Completed "The Power of the Atom"
Completed "Escape!"
Took the G.O.A.T.
Got the Pip-Boy 3000
Completed "Operation: Anchorage"
Completed "Paving the Way"
Completed "The Guns of Anchorage"
Completed "Aiding the Outcasts"
Completed "Free Labor"
Completed "Unsafe Working Conditions"
Found All 100 Steel Ingots
Completed "Into The Pitt"
Completed "Who Dares Wins"
Completed "Shock Value"
Completed "Death From Above"
Reached Level 30 with Good Karma
Reached Level 30 with Neutral Karma
Reached Level 30 with Bad Karma
Completed "A Meeting of the Minds"
Completed "Walking With Spirits"
Completed "The Local Flavor"
Discovered all locations within Point Lookout
Completed "This Galaxy Ain't Big Enough…"
Collected all Alien Captive Recordings
Completed "Among the Stars"
Completed "Not of This World"