A very simple modification that uses the already implemented label above all the living entities in the game but just no more used anymore. And depending if you consider this cool or cheating, you'll love or hate this mod. So, if you're gonna hate this, just don't install it.
How to install
Simply open your Minecraft.jar file and replace a class file inside by this one :
Because none of the classes for entities is forced to specify their initial value as a constant, I can't know the initial or maximum health possible value for sure and compare it with the health points of the entities and show a health bar. And personaly, even if a health bar is pretty, as a modder and tester I prefer numerical values to precisely mesure the impact of what I'm using (custom or not weapons, with and without critical hit effect, etc.) It's in this optic that I've made this mod and I was considered it useful for me and probably for others playing and testing Minecraft and mods for it.