A couple I don't see listed that are worth a gothrough Rock of Ages and McPixel.

Tiny and Big's definitely doable in a day.
Fun mechanics with a good soundtrack. I just wish there were more of it.

Portal was apparently already mentioned, good.

How about Mirror's Edge? The first time I played it on normal, it took me several weeks, but just because I was stuck to some places for many days, and restarted the game from the start once.

But when I recently replayed it on hard difficulty, I think it took less than a day. In fact it felt about the same to me length-wise as Portal, but can't be sure of course.

But then, I think most classic point&click adventures could also be played through in less than a day, if you know the solutions to puzzles... right?

Yes.

Spec Ops: The Line took me about five hours, I think.
I've heard that Scratches is quite short as well.
Beneath a Steel Sky.
5 Days a Stranger is pretty short; I can't imagine the rest of the series' games being much longer.
Both Portal games belong on that list.
Imscared, too.

The Walking Dead and The Wolf Among Us. The games are episodic and designed to be played as individual episodes, while every single episode takes about 2-3 hours of dedicated play. (both contain 5 episodes, so you could actually finish the entire thing in a single day if you really wanted. Then again, The Wolf Among Us only has the first episode out, so I'm judging by TWD.)

I think , you will get some biased answers , of course not for all games.

some games can be played in one day , sure , but if some users start to list game which they rushed , you lost some gaming experience in my point of view :/

also , it' depends how good you are too.

one exemple : Bastion ; when i finished it , it took me far more than one day; but i agree, i was not so good ;) , so some others could finish it in one day, that depends the last point i mentionned.

Episodic games are perfect for this topic, except if you're stuck :p

You should include games which can be played in a session ; like Shooters : ie Really Big Sky : great game, and fit pretty well if you don't have a lot of time. Well i think you got the picture :)

Anodyne - took me a weekend, but I was backtracking and reloading on deaths and generally not rushing to win. Highly awesome.
any of the Wadjet Eye games
any single episode of Deponia
Uplink
Trine

Nearly there:
FTL is kind of tricky, in that a playthrough takes less than a day, but starting it from stratch and not dying will probably take longer.
Rogue Legacy - same
Zafehouse Diaries - ditto
Book of Unwritten Tales - took me a weekend, people report shorter playthroughs.
Waking Mars - I personally got bored after it became a sandbox, YMMV.

Special mention:
Defender's Quest. There's a lot of content, but it will take less than a day to see most of the plot and the official good ending, and the game lends itself well to be played in very short intervals - missions are short, there's a cutscene archive, etc.

I'm not talking about deliberately hurrying through a game here, but rather games which are designed to be an experience which can be completed in one sitting. Same principle as a movie. Of course you can stop a movie at any point and resume it later, same as any game, but some games lend themselves to being played all in one go. That is what this list is intended to uncover.

I'm fine with suggestions where the main quest can be completed in one sitting, but where there is more content that can lead to further playing down the line. Or games where the replayability is high, so that another playthrough uncovers different things. Again, this list isn't meant to collate games where you can suck the joy out of them by rushing through them. It is for games which are a worthy way to spend what little gaming time one might have in their otherwise busy days.

Thanks for all the additional suggestions folks, I'll update the master list as well as the GOGmix.

Thanks, Zombies. is a great suggestion, that was a one-day game for me as well. I can't in good conscience list Rogue Legacy however, since personally I've been picking it up for stray hours over several evenings and still not defeated the first boss. Perhaps my skills just aren't up to it, but if you beat this in one sitting, you should think about gaming professionally. ;)

I still haven't beaten this one, keep losing based on stress level. I expect it could be completed in one go once one got good at it though. I'll put it on the list.

Yeah, there are plenty of games that can be beaten in a single sitting, if you know what you're doing and/or are willing to abuse exploits, glitches and skips. Case in point, you can beat both Baldur's Gates including expansions back to back in less than two hours. However, I take it we're looking for games where an average player will spend 5ish or less hours on their first playthrough.

Anyway, a couple of other games. Most, possibly all, of which have been mentioned already.

Mark of the Ninja - This one might be a bit over the 5 hour mark. Steam claims I've spent 15 hours, but that's playing through twice, hunting down all the secrets and such.
Super Meat Boy
Braid

And a comment about FTL. Being a roguelike, a single play from start to beat/death won't take more than a couple of hours, but with death being prevalent and luck playing a decent factor, it can take well over 5 hours to beat the game.

Thank you, you have grasped the intent of this list. I am not a fan of the speed playthrough, I am someone who wants to enjoy the time they are spending with the game as it unfolds.

Good points. Braid is on the list, and I will add the other two suggestions. I will pull FTL, as you are not alone in stating that a proper playthrough can take far longer.

Here is my addition to the list with the games I have finished lately in one-sitting-evening-session :-)

From GOG.com:
Zork: Grand Inquisitor (awesome game, highly recommended!)
Earthworm Jim 1&2
Neighbours from Hell 1&2
Dragonsphere
Lure of the Temptress
Teenagent
Leisure Suit Larry: Love for Sail! ;-)

From Humble Bundles:
Shank
NightSky
Jamestown
And Yet It Moves
Lugaru HD
Wizorb
Lone Survivor
Rochard
Shatter
Hammerfight

/e: ...and The Tiny Bang Story!

Wow, lots of additions! Thanks for the suggestions.

Thirty Flights of Loving is a wacky game that can be completed in 10-30 minutes...

I got it for a couple of bucks when it was on sale - don't overpay for it because it is way, way, way too short - but still it's a bit of wacky fun ;-) Thanks for the link!!

Adding another game - Inescapable. Took a bit over three hours of play to finish. It's an old-fashioned platform/shooter of the Sega/Amiga schools, though with a decidedly post-modern ending.

Though it can be easy to get lost in the later areas for a while, the whole thing is small enough that you eventually stumble back to where you need to be. And it's nice how you gradually accumulate new abilities that make difficult or impassable areas accessible.

A bit pricey for the playing time (I picked it up during the half-off sale), but still recommended. Quite a bit of fun.