I couldn't find anything that does exactly this, but please direct me to one if there is a mod like this.
Here's what the mod should do:
As soon as you damage a mob, a health bar shows up displaying its current health and the bar stays there until the mob dies or after a certain time period (maybe 10 seconds or so?). This could be useful for mobgrinders, farms, etc. For modders, use Modloader or Forge if you want, and the mod doesn't have to be compatible with anything except Optifine.
Please don't reply with anything along the lines of "I can't do this" or "maybe later", those posts are of no use to anyone.