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.
Killed 300 people
Got the Pip-Boy 3000
Took the G.O.A.T.
Completed "Escape!"
Completed "Galaxy News Radio"
Completed "The Waters of Life"
Completed "The American Dream"
Completed "Take it Back!"
Completed "Big Trouble in Big Town"
Completed "The Superhuman Gambit"
Completed "The Wasteland Survival Guide"
Completed "Those!"
Completed "The Nuka-Cola Challenge"
Completed "Head of State"
Completed "The Replicated Man"
Completed "Blood Ties"
Completed "The Power of the Atom"
Completed "Tenpenny Tower"
Completed "Strictly Business"
Completed "You Gotta Shoot 'Em in the Head"
Completed "Trouble on the Homefront"
Completed "Agatha's Song"
Completed "Reilly's Rangers"
Reached Level 8 with Bad Karma
Reached Level 8 with Neutral Karma
Reached Level 8 with Good Karma
Reached Level 20 with Neutral Karma
Killed 300 creatures
Won 50 Speech Challenges
Picked 50 locks
Discovered 100 locations
Kill all the Super Mutant Behemoths
Completed "Oasis"
Reached Level 20 with Good Karma
Completed "Finding the Garden of Eden"
Reached Level 20 with Bad Karma
Completed "Picking up the Trail"
Completed "Following in His Footsteps"
Reached Level 14 with Bad Karma
Reached Level 14 with Neutral Karma
Reached Level 14 with Good Karma
Completed "Stealing Independence"
Completed "Tranquility Lane"
Completed "Scientific Pursuits"
Placed a grenade or mine while pickpocketing
Completed "Rescue from Paradise"
Made one of every custom weapon
Hacked 50 terminals
Collected 10 Vault-Tec Bobbleheads
Collected 20 Vault-Tec Bobbleheads
Completed "Aiding the Outcasts"
Completed "The Guns of Anchorage"
Completed "Paving the Way"
Completed "Operation: Anchorage"
Completed "Free Labor"
Completed "Unsafe Working Conditions"
Completed "Into The Pitt"
Found All 100 Steel Ingots
Completed "Who Dares Wins"
Reached Level 30 with Neutral Karma
Reached Level 30 with Bad Karma
Reached Level 30 with Good Karma
Completed "Shock Value"
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…"
Completed "Among the Stars"
Completed "Not of This World"
Collected all Alien Captive Recordings