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