Optional "below 0 HP rule" for OSR and D20.
When character's HP reaches 0 or beyond roll save vs. death (or equivalent) - negative HP to roll. If the character fails he dies. If successes wait for the next hit and roll again with new negative HP modifier.
Example: Character's save is 15 and orc hits him gutting him to -2 HP. Player rolls d20 and get's a result of 17. The negative HP is decreased from the roll, so the final result is 15. Close call.
A couple of turns later the character is hit again with 4 points of damage, so his total HP is now -6. Player rolls d20 to save and his result is 17 again, but the character dies now (17 - HP -6 = 11).
Don't want to roll save versus? No problem. Roll against CON. Negative HP is decreased from CON and you try to roll below your CON to survive.
Character's CON is 15 and he suffers damage to -3 HP. Player rolls d20 against CON 15-3=12 and gets a result of 8. He survives the blow.
Next turn character is hit again and now his total HP is at -8 HP. Player rolls d20 against his modified CON at 7 and gets a result of 11. The character dies.
To keep combat fast don't use this rule for normal monsters and goons. They die at 0 HP (or are incapacitated). For "boss monsters" and other significant opponents you can use this rule though.
Character party encounters 6 hobgoblins and their shaman leader. 6 ordinary hobgoblins die normally at 0 HP but Referee wants the shaman leader to be a little tougher and meaningful opponent so he uses this rule.
If the game doesn't list monster CON use its save versus regardless of which version of this rule you use with player character. Or make up the CON depending on opponent's size, racial toughness in description, HD and how hard you want it to be.
I like the death and dismemberment chart a lot, but this is a bit simpler. The point is that character's are not in perfect health at 1 HP and die whether the damage is 1 or 100 points. More damage you suffer beyond 0 HP more likely you die.
There can be lots of optional rules for negative HP effects but I still like to keep things simpler. It's just that I like the idea that characters with positive hit points of any are in good enough condition but beyond 0 they start to get weaker and weaker.