The Diehards Handbook is supplemental manual filled with custom rules my gaming group and I have been using since the switch between AD&D to 3rd Edition. At its core, it is an alteration to how the Hit Point system works, but trust me when I say its much more than that; there are many more things to explore in the Diehards Handbook than just a revised Hit Point system. Anyways, read the wiki if you got 3-5 minutes. I think the wiki explains it in better detail than I ever could:
http://www.padnd.com/wiki/index.php?title=Diehards_Handbook
http://members.cox.net/meat/
Feel free to leave comments.
I should mention that my gaming group and I have been playing with these rules for a little bit over 5-years now and found they greatly increase the amount of fun we have in each session by "upping the ante". Feel free to give these rules a try for yourself, and feel free to check out the custom adventurers I made on my webpage.