My 3.6 rules incorporate some of 4th Edition's new combat rules, with a few extra modifications. The rules below can be modified by certain Feats.
Hit Points & Damage
A PC reduced to half their Hit Points, whether from melee or nonlethal damage, is considered Bloodied.
A character can only fight as long as they have the energy, after which Fatigue sets in. A Bloodied character may fight for an additional number of rounds equal to their CON ability bonus (e.g. a bloodied Character with a +3 CON Bonus can fight for three further rounds before incurring a penalty). After these number of combat rounds have passed whether in melee or not, a Character must make a Fortitude save (DC15) every round. If they fail they receive a -2 penalty to all attack rolls and Reflex saves for the remainder of the combat encounter. These rules also apply to monsters and NPCs.
Once per combat encounter, a Character can use their Second wind as a full action. They must be bloodied in order to do so. Use of Second Wind heals the equivalent of a 1/4 of the Character's total HP, including those lost to nonlethal damage. Healing surges may not restore damage caused by certain necromantic spells, undead, disease, and poison at the DMs discretion.
A Character has a number of Second Wind equal to their CON ability bonus +1. They can use their Second Wind that many times a day a day (i.e. between resting for at least 8 hours), once per combat encounter.