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 "The American Dream"
Completed "The Replicated Man"
Completed "Trouble on the Homefront"
Reached Level 8 with Bad Karma
Placed a grenade or mine while pickpocketing
Completed "Head of State"
Completed "Oasis"
Completed "Tenpenny Tower"
Completed "Agatha's Song"
Reached Level 20 with Neutral Karma
Reached Level 20 with Good Karma
Hacked 50 terminals
Collected 20 Vault-Tec Bobbleheads
Reached Level 14 with Good Karma
Reached Level 14 with Neutral Karma
Kill all the Super Mutant Behemoths
Won 50 Speech Challenges
Reached Level 8 with Good Karma
Reached Level 8 with Neutral Karma
Completed "Stealing Independence"
Completed "The Nuka-Cola Challenge"
Made one of every custom weapon
Completed "The Superhuman Gambit"
Discovered 100 locations
Collected 10 Vault-Tec Bobbleheads
Completed "Reilly's Rangers"
Picked 50 locks
Killed 300 people
Reached Level 20 with Bad Karma
Completed "The Wasteland Survival Guide"
Killed 300 creatures
Completed "You Gotta Shoot 'Em in the Head"
Completed "Big Trouble in Big Town"
Completed "Strictly Business"
Completed "Take it Back!"
Reached Level 14 with Bad Karma
Completed "Finding the Garden of Eden"
Completed "Picking up the Trail"
Completed "Rescue from Paradise"
Completed "The Waters of Life"
Completed "Tranquility Lane"
Completed "Scientific Pursuits"
Completed "Those!"
Completed "Galaxy News Radio"
Completed "Blood Ties"
Completed "Following in His Footsteps"
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"
Found All 100 Steel Ingots
Completed "Unsafe Working Conditions"
Completed "Into The Pitt"
Completed "Shock Value"
Completed "Who Dares Wins"
Reached Level 30 with Neutral Karma
Reached Level 30 with Bad Karma
Reached Level 30 with Good Karma
Completed "Death From Above"
Discovered all locations within Point Lookout
Completed "A Meeting of the Minds"
Completed "Walking With Spirits"
Completed "The Local Flavor"
Completed "This Galaxy Ain't Big Enough…"
Collected all Alien Captive Recordings
Completed "Among the Stars"
Completed "Not of This World"