The rules around death and dying vary quite a bit across editions. For as long as I can remember, I've used the general AD&D rule; 0 hit points and below is unconscious, and at -10 is death. Unconscious characters lose 1hp per round until treated or death is reached. The actual AD&D rule is a bit more nuanced - there's mention of permanent injury and scarring in the DMG at -6, a 'severe blow' death below -3. Like most things in AD&D, nothing is simple. For simple, you've got Moldvay and classic D&D - zero or lower is dead.
I remember an old post by Cyclopeatron that had Gary Gygax's house rule - I'll dig for the link - Gygax House Rules. Normal men died at zero hit points, but a classed character would be unconscious equal to his level +1. A level one character could survive and be unconscious to -2, a level eight character could survive to -9. Another house rule I've seen is unconscious at zero, dead at anything below zero.
Seems like a good time for a poll, especially since I'm about to ditch the -10 hit point house rule. A new poll is up: "How do you handle death, dying and zero hit points in your game?"