Final Fantasy VII was probably the worst for me, school holidays came around, 6-8 weeks straight off with nothing to do but just play constantly. Would often find myself on for 16 hours straight. Realised it was getting bad when I would wake up and get straight on at around 8 or 9am, then get the call for dinner at 6pm and have not eaten anything yet, or moved since I first sat down (except for possibly a quick bathroom break). Ahhh, the days before annoying requirements of life like "a job" and "friends" got in the way of proper gaming.
Most recent game I was hooked on was Half Life 2. First day I started playing it I figured a few hours gameplay to get the idea would be good, and finish it on the weekend. Found myself constantly saying "just a few more minutes, just to the next area" and putting off heading to bed. Last looked at the clock at around 10pm. Next thing I know, just as I beat the finale, there's a surprising amount of light coming in the windows, and there's birds I can hear whistling the morning's hellos. Got to sleep at about 5:30am, had to be up around 7am, but totally worth it.
Does anyone else find that when playing Sandbox games particularly, it's easy to waste a few hours doing nothing really at all? Just killing randoms, stealing cars, creating little self-made challenges, etc.