DubConqueror: Luckily I find more self-discipline as I'm getting older. In my thirties, when I just started PC gaming, I've sometimes spend a whole night continuously gaming, first culprit was Age of Empires I, but also Age of Kings, Battlefield 1942, CivCity Rome.
I got to know Baldur's Gate some 5 years ago and lost some night's sleep on that game as well on Icewind Dale. The recent dangerzone is Fallout 3, it's often hard to stop once I start.
Several times I thought I had matured out of excessive gaming 'binges' as it were, but each time something came along that shot that theory to hell. Only last year I decided to give KOTOR a go and couldn't stop until I was done. This year I lost 1.5 weeks to Kerbal Space program already when I really had to be prepare for a very important talk :P
Age of Empires I - I remember how impossible good the graphics was back then. Brought my 486 to a crawl though
HereForTheBeer: Pool of Radiance when I bought it shortly after release. Ditto Railroad Tycoon.
And geez, Battlefield 1942. Sooooo many marathon hours playing that gem.
Ah, I never went on a Battlefield marathon. I did go on several TF2 ones though :P Regardless I would dearly like to go on a Star Wars Battlefront marathon if people are still playing that, or Galactic Battlegrounds (someone reminded me of that game recently)