Return to Castle Wolfenstein.

Hm... I remember many games when I'm start play and didn't play another game till I'm end this game. Max Payne Trilogy and Half-Life 1 and 2 with addons. Batman Arkham series, Sleeping Dogs and Saints Row: The Third. But of course best are Deus Ex 1 and 3. I'm even remember what I'm sitting on computer without turn it off complete Deus Ex 1 two times.

binding of isaac, still trying to get the dark boy achievement :/

You know, OP. I've never got round to completeing Startopia. I've started it twice, got about 2/3rds of the way through, then, I dunno, my interest sort of petered off.

Team Fortress 2. Recently started playing again, wanted to play for about half an hour yesterday, ended up playing for 3 hours straight.

Jade Empire, Oddworld: Stranger's Wrath, and Darksiders all had me playing each one from start to finish in one playthrough straight. Jade Empire took me about a day and a half. Stranger's Wrath was 14 hours or so and Darksiders was about 27 hours or so.

Dota 2. It eats time like hell. Too bad that it has no ending. In fact it's not even released...

I don't think I've played any single player campaign/story in one sitting, i need to do something else in between sessions.
Though, i have played a few games for long periods of time, trying to complete them. Examples are Grand Theft Auto games, DOOM 1 and 2 and Max Payne 1 and 2.

Torchlight 2.

Final Fantasy Tactics

cs 1.6, 9 years and counting =)

Civ V. I just played Baldur's Gate and I have to restart the game with new character about 5 times already because I have no idea what I'm doing.

Fallout... any of them... i play 150 hours of New vegas in 2 weeks... that is 10+ per day.
also Borderlands and torchligth... well, any game wit loot...
I NEED TO GET MORE LOOT

NEED. MORE. LOOT

MORE LOOT

Armored Core, Bioshock 1, F.E.A.R. 2, Ratchet: Deadlocked, Red Faction Armageddon, Torchlight 2

Orcs Must Die 1 and 2!

I always end up playing the entire episode of The Walking Dead on the day of release. Can't help it!

Metal Gear Solid 3, Binding of Isaac, Terraria by the time, some FPSs (for their short life span), Fallout 1 and 2, Baldur's Gate, Icewind Dale, Planescape Torment, and a lot more.

Whenever I buy a Pokemon game, I have a really hard time to put it down.

When I bought Diablo 3, I played it for almost 76 hours, within 4 days. I stopped playing after that, I think I got sick of it.

I use to play Mobas for hours on end, but not so much any more.

Also, I invested a lot of time in Red Dead Redemption's multiplayer. It was a lot of fun playing with friends.

World Of Warcraft every new expansion i spend 5 days straight playing it. What a wonderful life i have.
Also the 2nd Starcraft 1 expansion i played through on all diffs in one sitting. same with Diablo: Hellfire

Dragon Age: Origins

The Binding of Isaac

Portal, Space Pirates and Zombies (SPAZ), Star Wars: Knights of the Old Republic, Star Wars: Knights of the Old Republic II

i played dota 2 for 70 hrs in a week

Prince of Persia: The Sands of Time