Inspired by 4E
So, I have already started house ruling stuff as inspired by 4E (despite my negative feelings about the tone and power of the new rules, I still think there are great ideas in there) and in some ways, rules were were already using run parallel to some of the ideas, so it works out.
For example, since hit points have always been (at least as far as my group and I see it) some abstract combination of physical toughness, quickness, and fighting ability, we long ago made natural healing determined by BAB. That is, it goes up with levels and those who are better fighters increase faster. So, for example, in Aquerra games every 8 hours of light activity gets you back a number of hit points equal to your BAB +1. The heal skill and bed rest could be used to increase this rate.
A couple of months ago we instituted our version of the "Second Wind" rule (which we created before knowing how it will work in 4E and preferring ours now that we know) which allows a character to take 10 minutes of rest or light activity to make a Con check against DC 11 to regain a number of hit points equal to the character's BAB + Level. This cannot be invoked after taking damage from a trap or hazard, but trap or hazard damage can still healed in this way after a combat (as to not have the book-keeping of determining the source of damage).
Second wind cannot be used if a character is exhausted or at negative hit points.
Also, we have just replaced our rules for death and dying with something we feel is more elegant and similar to the suggestion made in one of those 4E Design & Development articles for trying 4E stuff with 3.5 games.
You still have an at death's door negative range from -1 to the negative equivalent of 10 + base fort + Con mod, which is what we were playing with already. However, each turn, while dying you roll d20 modified by Con. If the result if 1 thru 10 you bleed (losing a point), if the result is 11 thru 19 you stay the same. If the result is a 20 or higher you stabilize. A natural '1' always bleeds, a natural '20' always stabilizes. If you reach the end of your negative hit point range, OR if you ever 'bleed' three turns in a row, you die.
If you are being treated by someone with healing while dying you may use the healer's skill bonus as a bonus to your check each turn INSTEAD of your CON - otherwise healing works the same (and if you bleed three times in a row or reach the neg hp total during this time you still die). Note: In Aquerra games, performing first aid on someone who is dying takes 10 rounds, 5 rounds if you have aid from someone else who make an aid other check (in addition to the normal +2 bonus to the check they'd provide). Damage taken at negs is reduced as normal, only killing you if it brings you past your negative hit point total.
Using cure spells on someone at negative hit points works as normal (i.e. stabilizes and does a # of points which may or may not bring you to consciousness). Also note, we play that if you come back from negative hit points by means of cure spells short of heal as opposed to natural healing and rest, you are exhausted - as the mortal form was not meant to be brought back and forth from the brink of death so rapidly and so often.
Anyway, I am sure that as more 4E rules comes my way or when I finally get my hands on the books there will be other rules I will want to adapt. I am also intrigued by the skill system Pathfinder is using, for the simplicity of coming up with skills for NPCs (never had any complaints from players about skills).
Have you incorporated anything into your current games?