The Pathfinder game I'm scheduled to join in a few weeks has one of these. I've reproduced it below. (©2010 Vivian Abraham)

Hit Points, Conditions, and Healing

Conditions, or what happens when you are knocked below zero hit points

Hit points represent your character's ability to avoid lethal damage. When you reach zero hit points or go negative, you gain a negative condition at the discretion of the GM.

For example:

Grignr the Barbarian faces off against Sargon the Sorcerer in the public arena, a dispute over the ownership of some gem. Grignr fells Sargon with one mighty blow of his axe, slamming him to the ground. Sargon's leg is broken, reducing his move rate and giving him -2 on all rolls, including his Concentration check to cast, due to the excruciating pain. Sargon musters his strength, makes his concentration check, and casts a maximized sound burst on the hapless Grignr, knocking him below zero. He makes his Fortitude save against the stun effect of the spell, but now has a concussion, lowering his rolls by 4, including his initiative while he desperately attempts to clear his head. Grignr aims another mighty blow at Sargon, critting again, and neatly splits him in half.

Healing

Hit points heal fast. With an hour's full rest, you'll get them all back. Conditions require more work. Simple conditions like a light fracture or a minor burn can be alleviated over time with an appropriate Heal check or immediately with a cure light wounds. More serious conditions require more time, better Heal checks, or stronger healing spells.

Healing spells still cure the same number of hit points.

For example...

The spell cure serious wounds has two effects. First, it can be used to cure a number of hp. Second, it can be used to remove certain negative conditions, such as a broken bone. If someone has no hp and has a broken bone, cure serious wounds will both heal the broken leg and heal hp. If a cure moderate wounds is used, it will only heal the hp damage, not the broken leg. Proper application of the Heal skill, together with rest, can downgrade negative conditions over time. So if you broke your leg a few days ago, had it set and splinted, and haven't been walking on it, it could be downgraded to a moderate wound.

Sample Conditions

• Black eye / swollen eye / blinded
• Ringing in your ears / deafened
• Stunned / Concussed / Unconscious
• Light fracture / Broken Bones /
• Compound fracture Minor blood loss / serious blood loss / critical blood loss
• Minor burn / serious burn / critical burn
• Wheezing / choking
• Exhausted / slowed / paralyzed
• Dead

So, to sum up...

1. There are no negative hit points.
2. Once you run out of hp (or an attack takes all of your hp, plus some), you gain a negative condition of the GM's choice.
3. Conditions are cumulative, and will lead to character death unless they are tended to.
4. There is no complete list of conditions, as characters have such unique methods of getting in trouble.
5. NPCs will most often receive the condition "dead".
6. The Heal skill treats conditions and helps you determine their severity, and therefore what spell is best suited to tend to it.
7. Hit points heal fast. Conditions take treatment and time, or the appropriate level spell.

Side note - feats and abilities that affect negative hit points will need adjustment. For example. Die Hard now allows you to ignore a single negative condition.