Here's a little system I've worked up, being inspired by the FightingFantasist's post:
Wound Points and Fatigue
Hit points are now kept track of in two ways:
First, you have your Wound Points (WP). This is your normal Maximum HP minus any Wound Points you have suffered from damage. Wound Points are the minimal amount of damage dealt by an attack. If an attack deals 1d4+1 damage, it deals 2 Wound Points. Critical Hits deal their maximum damage as full WP damage (thus dealing a like number of FP).
When you reach 0 WP, you must make a saving throw versus Death, or you die. If you succeed, you are at 1 WP and 0 FP and are unconscious. Every time you reach 0 WP, you lose 1d3* points (d6: 1-3= 1, 4+5= 2, 6= 3) from a randomly-determined attribute (d12: 1-5= Con, 6= Str, 7= Dex, 8= Int, 9= Wis, 10-12= Cha).
Second, you have Fatigue Points (FP). Your maximum FP is equal to your current WP. When damage is dealt from an attack, your FP takes the full damage rolled; thus, that 1d4+1 damage, when rolled as a total of 4 points damage, does 4 points to your FP and 2 points to your WP.
When you reach 0 FP, you fall unconscious. If you are left alone without first aid, every turn you must roll a saving throw versus Death. If you fail, you remain unconscious; if you succeed you wake up with 1 FP. If you roll a natural 1 or miss the save by 10 or more, you suffer 1 WP.
After a battle is over, you may take a full turn (10 minutes) to bandage your wounds and rest. If you do so, your FP are restored to their maximum level (your current WP).
Traps also deal their damage in FP and WP; like damage suffered in battle, you must bandage and rest immediately to regain all FP.
If you do not bandage and rest immediately after suffering damage, you may still regain FP by subsequently resting for one turn. Each turn you rest you regain 1 FP or a number of FP equal to your Constitution bonus to HP, whichever is greater.
WP are regained through the standard HP healing process.
Spells such as cure light wounds, cure serious wounds, cure critical wounds, heal, and potions of healing and extra-healing cure both WP and FP; both rise by a like number equal to the number rolled.
Other Uses of Fatigue Points
Clerics may expend FP to restore FP in other characters and to empower his turn undead ability. A cleric may expend a total number of FP in this way equal to his level. To restore FP the cleric must touch the target; he may channel a number of FP per round to the target equal to his level. He may improve his chance of turning by +1 for every 2 FP expended in this way. He may also increase the number of HD affected by expending 1 FP per HD to be increased. These last two must be expended BEFORE the turn check is made. Thus, a 3rd level cleric can channel 3 FP to a target each round, or increase his turn roll chance by +1, or increase the number of HD affected by 3.
Fighters may expend FP to improve their chances to hit and deal damage. A fighter may expend a total number of FP in an attack equal to his level. He may divide this into a bonus to hit and a bonus to damage. For every 2 FP he gets a +1 to hit, for every FP he gets a +1 to damage. FP for either bonus must be expended before the roll to hit! Thus, a 7th level fighter may expend 7 FP in an attack, spending 4 on a bonus to hit and 3 on a bonus to damage, for a total of +2 to hit and +3 damage.
Magic-users may expend FP to power their spells. A magic-user may expend a total number of FP in casting a spell equal to his level. He can use FP to increase his effective casting level with the spell; every increment of FP equal to the spell level increases his effective casting level by one. He can also expend FP to give a target of a spell a penalty to his saving throw; for every 2 FP expended this way the target suffers a -1 penalty to save (1 target per 2 FP). He can use his own fatigue to completely empower a memorized spell, such that the memorized spell is not lost when cast; this costs 3 FP per spell level. Finally, he can use FP to attempt to counter an opponent’s spell as it is cast. Counter-spelling costs 1 FP per level of the spell to be countered. Use of this ability must be declared before initiative is rolled; even if the magic-user’s side loses initiative, he still gets the chance to try to counter-spell. If the spell being cast is one he knows, he will automatically know the spell level being cast; if he does not know the spell, he may make a saving throw against Spells to correctly identify the spell being cast. He must make a Chance to Learn Spell check, with a penalty of 5% per level of the spell being cast, to successfully counter the spell. If he does not expend enough FP to match the level of the spell, the attempt automatically fails (he gains no benefit from expending too much FP).
Thieves may expend FP to increase their chances of success with their thieving abilities. A thief may expend a total number of FP to improve an attempt to use a thieving ability equal to his level. For every FP expended before attempting the ability, the thief adds 1d6% to his chances of success. Note that after all modifiers are accounted for, no ability can ever be greater than 99%. Thus, a 3rd level thief can expend 3 FP to improve his chance to Pick Pockets by 3d6%.