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