Short Steam Games

Windosill.

Spec Ops: The Line - About 6-8 hours.
Half Life 2 - About 10 hours
Bad Rats - 30 seconds (technically its longer but nobody has gotten further than this)

'Hamlet or the Last Game without MMORPG Features, Shaders and Product Placement' is shorter than its title.

Dinner Date - 20 minutes.

thirty flights of loving

Home front- sub 5hrs on normal without skipping cutscenes.

Eversion <3

Lone Survivor

Shank and Shank 2

TRAUMA and To the Moon.

I'd argue Thomas Was Alone is pretty short, but it's also very great. I also second the notion for Spec Ops.

The Walking Dead - around 15 hours.

DLC quest - If it ever gets Greenlight is less then an hour

Samorost 30 minutes
Samorost 2 1 hour
Machinarium 4 hours
Botanicula 3 hours
Unmechanical 3 hours
The Cave 3 hours for each playthrough

Braid.

Short, but worth it,

These ar some short game that I have played:
1000 Amps,
Antichamber,
Botanicula,
Eversion,
NightSky,
Q.U.B.E. and
Windosill

Dear Esther

SONIC THE HEDGEHOG 4 Episode I & II. You will most likely finish them in one session.

The Darkness 2, but it's all awesome. :D
Containment: The Zombie Puzzler.
Sequence but it's awesome too.
They Bleed Pixels... depending on how good you are I guess.
Portal, but... you probably guessed I was gonna say it's awesome... :P

Home is certainly short, yet strangely enjoyable!

The Graveyard and THirty Flights of Loving. Both are extremely short (5 minutes and 15 minutes respectively).

LIMBO is long if you're bad at puzzles (like me)

Castle Crashers is incredibly short, especially first run with 2-4 people

Trine 1 can be pretty short if you're just going for plot, not 100%

BIT.TRIP BEAT is short on easiest difficulty

Sword & Sworcery isn't too long, maybe 4-5 hours if you do the Moon Grotto

EDIT: Portal 1 is very short and simple

EDIT2: Eversion. Recommended to all hell.

The Cat and the Coup is another very short one which hasn't been mentioned yet.

It's also free :)

To the Moon...but it's great!