Achievables #1: Commander Keen

Seeing as how Steam has already released Episodes 1-5 of Commander Keen as a single pack, I'll treat Keen as a full 1000-gamerscore release. Oh, and because it's awesome, I'm including Episode 6: Aliens Ate My Babysitter as well.

from Episode 1: Marooned on Mars:

Boing
5 - Acquire the Pogo Stick.

You're In Control
25 - Recover the Joystick.

Charged Up
25 - Recover the Battery.

This Sucks
25 - Recover the Vacuum Cleaner.

Clean-Burning Fuel
25 - Recover the Everclear.

Voices in my Head
10 - Listen to all the Yorp Statues.

Dark Side of Mars
10 - Teleport to the dark side of Mars.

Martian Surprise
25 - Discover the Secret City on Mars.

from Episode 2: The Earth Explodes:

Who Turned Out The Lights
5 - Use a lightswitch aboard the Vorticon Mothership.

Respect your Elders
10 - Listen to the two Elder Vorticons in stasis.

London is Safe
15 - Disable the Tantalus Ray aimed at London.

Paris is Safe
15 - Disable the Tantalus Ray aimed at Paris.

Rome is Safe
15 - Disable the Tantalus Ray aimed at Rome.

Cairo is Safe
15 - Disable the Tantalus Ray aimed at Cairo.

New York is Safe
15 - Disable the Tantalus Ray aimed at New York.

D.C. is Safe
15 - Disable the Tantalus Ray aimed at D.C.

Sydney is Safe
15 - Disable the Tantalus Ray aimed at Sydney.

Moscow is Safe
15 - Disable the Tantalus Ray aimed at Moscow.

from Episode 3: Keen Must Die!:

I Am So Lost
10 - Use the teleporters on Vorticon VI 50 times.

It's not a Myth
25 - Ride the Lock Mess Monster to the secret level.

I Am Invincible
15 - Pick up an ankh.

Filicide
10 - A vorticon mother aimed a fireball at you and hit one of her own children.

The Big V
50 - Acquire the highest honor the Vorticon race has to offer.

from Episode 4: Secrets of the Oracle:

One Council Member
10 - Rescue one council member.

Quorum
25 - Rescue four council members.

This Meeting Will Come to Order
40 - Rescue all eight council members.

I CAN swim!
15 - Find the Wetsuit.

Swim Swim Hungry
5 - Narrowly avoid being eaten by the second-dumbest creature in the universe.

They'll Be Afoot
25 - Discover the Pyramid of the Forbidden.

Number Nine?!?
40 - Rescue the council's janitor.

from Episode 5: The Armageddon Machine:

Lower Level Breached
20 - Reach the Security level of the Omegamatic.

Upper Level Breached
30 - Disable all four devices guarding the upper level of the Omegamatic.

QEDestroyed
50 - Destroy the Quantum Explosion Dynamo.

The Planet Below
25 - Discover the secret base on Korath III.

Long Way Down
25 - Collect all the vials of Vitalin on Korath III in one jump.

from Episode 6: Aliens Ate My Babysitter!:

Beneath the Bean-With-Bacon
5 - Discover the secret tunnel beneath Keen's ship.

Cliffhanger
10 - Cross the cliff on Fribbulus Xax.

Om Nom Nom Nom
15 - Collect the second biggest sandwich Keen's ever seen.

Rocket Man
30 - Take the rocket to the Bloog Space Fortress.

Don't Tell Mom The Babysitter's Alive
45 - Rescue Molly.

To The Stars!
25 - Catch a ride to the Blooglab Space Station.

from no game in particular:

Now I Know My ABCs
25 - Learn the Galactic Standard Alphabet.

The Cake is Not a Lie
20 - Collect 100,000 points from cakes and pastries.

Brain Freeze
20 - Collect 100,000 points from frozen treats.

Sugar Rush
20 - Collect 200,000 points from sugary snacks.

Fast Food
20 - Collect 100,000 points from pizza and burgers.

Burp!
20 - Collect 150,000 points from sodas.

Achievement Unlocked
10 - Open 50 doors with keycards or gems.

The Enemy is Toast!
10 - Fire off 1000 shots.

The Many Secret Lives of Billy Blaze
10 - Collect 2000 total lifewater drops, Vitalin vials, and/or vivas.

Points After Death
10 - Collect an item after you die.

Total: 1000

BLEEP BLOOP.