Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Bone Breaker (5 points): Kill 30 enemies with thrown physics objects.
Deadly Harvest (5 points): Kill an enemy by planting a hopper mine.
Hack Attack! (5 points): Kill five enemies with a Manhack.
Hot PotatOwned (10 points): Kill a Combine soldier with his own grenade.
Conservationist (5 points): Kill five enemies with the same energy ball.
Think Fast! (10 points): Kill an Elite Soldier with his own energy ball.
Grave Robber (5 points): Steal a Zombine's grenade.
Zombie-que (5 points): Use flares to light 15 zombies on fire.
Malcontent (5 points): Escape the apartment block raid.
What cat? (5 points): Break the mini-teleporter in Kleiner's lab.
Trusty Hardware (5 points): Get the crowbar.
Barnacle Bowling (5 points): Kill five barnacles with one barrel.
Anchor's Aweigh! (5 points): Get the airboat.
Heavy Weapons (5 points): Get the airboat's mounted gun.
Vorticough (13 points): Discover the hidden singing vortigaunt cave in chapter Water Hazard.
Revenge! (10 points): Destroy the hunter-chopper in Half-Life 2.
Blast from the Past (10 points): Find the HEV Suit Charger faceplate in Eli's scrapyard.
Zero-Point Energy (5 points): Get the Gravity Gun in Black Mesa East.
Two Points (2 points): Use DOG's ball to make a basket in Eli's scrapyard.
Zombie Chopper (25 points): Play through Ravenholm using only the Gravity Gun.
OSHA Violation (5 points): Kill 3 enemies using the crane.
Targetted Advertising (5 points): Pin a soldier to the billboard in chapter Highway 17.
One Man Army (5 points): Destroy six gunships in Half-Life 2.
Keep Off the Sand! (20 points): Cross the antlion beach in chapter Sandtraps without touching the sand.
Bug Hunt (10 points): Use the antlions to kill 50 enemies.
Flushed (5 points): Kill an enemy with a toilet.
Warden Freeman (10 points): Survive the second turret standoff in Nova Prospekt.
"Radiation Levels Detected" (5 points): Get through the toxic tunnel under City 17 in Half-Life 2.
Plaza Defender (10 points): Survive the generator plaza standoff in chapter Anticitizen One.
Counter-Sniper (5 points): Kill all of the snipers in City 17.
Fight the Power (10 points): Shut down the supression device by disabling its generators.
Giant Killer (10 points): Survive the rooftop strider battle in the ruins of City 17.
Lambda Locator (15 points): Find all lambda caches in Half-Life 2.
Watch Your Head! (5 points): Make it to the bottom of the Citadel's main elevator shaft in one piece.
Containment (5 points): Contain the Citadel core.
Pacifist (10 points): Contain the Citadel core without killing any stalkers.
Car Crusher (5 points): Use the cars to squash 15 antlions in Half-Life 2: Episode One.
Elevator Action (10 points): Survive long enough to get on the parking garage elevator.
Live Bait (10 points): Help Alyx snipe 30 enemies in Half-Life 2: Episode One.
Attica! (5 points): Destroy the gunship in the hospital attic.
Citizen Escort (15 points): Don't let any citizens die when escorting them to the escape train.
Escape From City 17 (20 points): Escape City 17 with Alyx.
The One Free Bullet (40 points): Beat Episode One firing exactly one bullet. Grenade, crowbar, rocket, and Gravgun kills are okay.
Acid Reflex (5 points): Kill an Acid Antlion worker.
Get Some Grub (20 points): Squish every Antlion Grub in Half-Life 2: Episode Two.
Pinata Party (5 points): Find and break every web cache in Half-Life 2: Episode Two.
Into the Breach (5 points): Help Griggs and Sheckley hold off the Antlion invasion inside the mine shaft.
Hit and Run (5 points): Run over 20 enemies with the car in Half-Life 2: Episode Two.
Puttin' On a Clinic (15 points): Defeat the chopper in Half-Life 2: Episode Two without any misses.
Cache Checker (10 points): Find every radar cache in the Under The Radar chapter.
Gordon Propelled Rocket (5 points): Unlock the rocket launcher lambda cache in the Under The Radar chapter.
Pedal to the Metal (5 points): Beat DOG in a race to the White Forest base.
Little Rocket Man (30 points): Send the garden gnome into space.
Neighborhood Watch (35 points): Save all buildings outside the missile silo from destruction.
Payback (10 points): Kill a Hunter with its own flechettes.
Lab Rat (5 points): Acquire the fully powered Aperture Science Handheld Portal Device in Portal.
Fratricide (5 points): Do whatever it takes to survive in Portal.
Partygoer (5 points): Make the correct party escort submission position decision in Portal.
Heartbreaker (10 points): Complete Portal.
Terminal Velocity (5 points): Fall 30,000 feet in Portal.
Long Jump (5 points): Jump 300 feet in Portal.
Cupcake (10 points): Beat two Portal advanced maps.
Fruitcake (20 points): Beat four Portal advanced maps.
Vanilla Crazy Cake (30 points): Beat all six Portal advanced maps.
Basic Science (10 points): Earn bronze medals on all Portal challenges.
Rocket Science (20 points): Earn silver medals on all Portal challenges.
Aperture Science (40 points): Earn gold medals on all Portal challenges.
Turret (5 points): Accumulate 10 turret kills with a single turret.
Nemesis (5 points): Get five revenge kills.
Hard to Kill (10 points): Get five kills in a row without dying.
Master of Disguise (15 points): Trick an opposing medic into healing you.
With Friends Like these... (10 points): Play in a game with seven or more players from your friends list.
Dynasty (10 points): Win 20 games.
Hardcore (15 points): Accumulate 1000 total kills.
Powerhouse Offense (5 points): Win 2Fort with a shutout.
Lightning Offense (10 points): Win Well in 5 minutes or less.
Relentless Offense (20 points): Win Hydro without giving up a capture.
Impenetrable Defense (10 points): Successfully defend Dustbowl without giving up a capture.
Impossible Defense (30 points): Successfully defend Gravel Pit without giving up a capture.
Head of the Class (5 points): Play a complete round with every class.
World Traveler (5 points): Play a complete game on every map.
Team Doctor (5 points): Accumulate 25000 heal points as a Medic.
Flamethrower (5 points): Set five enemies on fire in 30 seconds.
Atomizer (10 points): Disintegrate 15 soldiers by throwing them into a Combine ball field.
Follow Freeman (10 points): Gain command of a squad of rebels in the uprising.
Grey Matter (5 points): Get 25 headshots as a Sniper.
Camera Shy (5 points): Detach security cameras from the walls.
Friendly Fire (5 points): Knock down a turret with another turret.

Additionally, there are eleven secret achievements:

Defiant Achievement (5 points): At the beginning of Half-Life 2 where the carbine cop asks you to pick up the can, pick it up and hit him with it.
Submissive (5 points): Put the can in the trash in Half-Life 2.
Where Cubbage Fears to Tread (5 points): Defend Little Odessa from the gunship attack in Half-Life 2.
Hallowed Ground (5 points): Escort Gregori safely through the church cemetery in Half-Life 2.
Singularity Collapse (25 points): Destroy the Citadel's reactor core in Half-Life 2.
Twofer (5 points): Defeat both antlion guards outside the White Forest.
Meet the Hunters (10 points): Survive the Hunter ambush with Alyx.
Gunishment! (5 points): Destroy the Combine cannon in the junkyard.
Quiet Mountain Getaway (10 points): Survive the ambush at White Forest Inn.
Secondary Silo Secured (5 points): Secure the launch doors on missile silo 2.
Defense of the Armament (20 points): Save the missile silo from the Combine offensive.

Half-Life 2