Fever_Discordia: You could get through the 5 levels of A.R.E.S: Extinction Agenda in a day - I think it was one of these planned episodic things that never made it to Ep2
http://store.steampowered.com/app/92300/
I just checked it out (it's DRM-free on GG, by the way). Seems like it might be brief enough, but I'm trying to avoid episodic games. Even those that will never have another episode. :) That's why I've left other titles, like Insecticide, off the list too.
zeffyr: Please, remove Fahrenheit from the list. This is definitely a game for at least couple of days. And from my point of view it has some replayability factors as well :-)
Funny you would mention this, I was just thinking about this game as I was reviewing the list. It really does take quite a while to finish, especially with all the dying and restarting. So I think I will pull it down, as "must have a perfect play-through to finish in a day" shouldn't be part of the criteria.