Well, obviously a site (mostly)filled with old games is going to have a high percentage of old gamers.

Been awhile since I've truly played one game for 8 hours without at least getting distracted a bit, but some of my highlights include....

Persona 3 (Just starting 4 Golden now ;D)
Final Fantasy anything
Heroes of Might and Magic III, especially in multiplayer.
Any Mario Kart or Smash Bros with a group or online.
Battlefield
Oh and 9 Hours 9 Persons 9 Doors (999)... That game will not let you put it down!

I don't know if I should be posting as it's pretty embarrassing, but I actually played the original "The Sims" to the point of dehydration over a break from work of a few days. It was around the time of the "Vacation" expansion on Mac (a good solid year or EP behind the PC version.) I actually collapsed at the top of the steps and lost vision for a minute or two and was taken to the hospital and given an I.V.

I could play any game I really like for 8 hours. Games usually fall into three categories for me: those that don't hold my interest for 10 minutes, those that are OK for about an hour, and those that hold my interest indefinitely.

I will agree with the other posters who have suggested that there is probably no game that ought to be played for 8 hours straight.

Most any game I really like I have played for 8 hours straight or more at one point or another.

If I ever find my old copy/get a new one, that would be awesome. :) (and, uhh, I'm no better — it was pure, blind fun for me).
OH YEAH THIS. Also oh my god it is so hard to find other people who played and adored this game. I think I stayed up until about 3am one night. Which would have been about a 6 hour session. So actually, that is the most time I've spent on a game.

Have you played Zero Escape yet? Also, the Vita living up to the purchase for you?

Well, you can't count bathroom breaks. In that case I'm out of the running. But I have forgotten to do other stuff, like eat, drink, sleep.

Nope, haven't started Zero Escape yet, still need to get the true ending for 999 first. :P

As for the Vita... Yes, it certainly is. I got it on Black Friday off Amazon for $180 with a few bonuses, and have been having a blast since. It + my 3DS make me a very happy portable gamer.

Oh, how many endings have you got so far?

I envy you so much for being able to get both. :P I've been tossing between the two for a long time, might be edging slightly more toward the 3DS (but Persona 4!!), but in the end I'm just trying to eke as much life as I can out of my DS Lite because the choice is too hard. :P Either way it's an intimidating amount of money to be shelling out.

I installed "Freedom Force" early on a weekend, and then for that entire weekend did almost nothing else besides eat, sleep, and play the game until I had completed it. I don't even know how many consecutive hours went into the longest of those sessions, but I'm sure that eight hours is an extremely conservative value.

Despite being a life-long gamer, I was a little scared at how addictive this one had proven itself to be, and I knew that if I started a new game of it the same thing would happen, so I actually uninstalled it at the end of that weekend as a preventative measure, which is something I'd never done with any other game (and since then I believe Mind Control's "Oasis" is the only other game I've ever uninstalled in order to stop myself from playing it).

Anything entertaining enough. I think SpaceChem was the last game I played that really made me lose track of not only time, but date as well.

Wow, that's pretty serious.
Did you have to actually them the reason why you got dehydrated? Or did you make up a story? :p

For everyone out there, healthy tip when you play for long or not: - every one hour or so, get up and walk a bit/stretch and go get a drink (water etc). Simple and good for you (and you can keep on playing even more! Tho I shouldn't encourage it :p)

Frequently held all day Elder Scrolls Oblivion Marathons with my best friend in 2009 & 2010.
Lost track of time frequently as well. Only visited the living room for food and drinks.
Chugged at least 2 big bottles of Coca Cola and Hero Cassis a day.

I don't think I've played more than 4 or 6 hours straight except once. I remember once when I had a lot of energy to speedrun Super Metroid and I finished it in one sitting, it took 9 hours and I don't recall eating, drinking or going to the bathroom. Other than that I always take small breaks and keep the primitive needs satisfied. It's annoying to concentrate if you're hungry, thirsty, tired, horny or need to go to the bathroom.

Potentially anything that interests me. Usually RPGs, because they suck you in.
The last time I went REALLY overboard was when I played Fire Emblem: Path of Radiance again. I got through it in one sitting. 24 to 28 hours, after that I felt like a corpse must feel. Just fell in my bed, drained and sleep deprived. Was good fun. Ofc I ate, drank etc. etc
but I haven't left the house or stopped for more than a few minutes, that's for sure.

You shouldn't play for that long, quite unhealthy, move around a bit, get some fresh air.
These days it is VERY hard for me to focus on a game because I am at the PC most of the time. You get distracted so easily, it's a pain in the ass. The only time where I don't feel like I'm missing out on something is when I keep him shut down on purpose and play on consoles and even that is difficult to do. The Internet is a dangerous time sink. More often than not you won't even know what you did most of the day...

StarCraft 1/2, Team Fortress 2 and the first Ghost Recon + 2 expansion packs.

What game should you play for 8 hours straight?
None

What game have you play for 8 hours straight?
Operation Fllashpoint MFCTI 1.16 ****Favorite****
Arma CRCTI
Arma 2 CRCTI AI