Bukkit

I don't have much experience with health plugins (so if I make erroneous assumptions, please correct me), but I've noticed that many of the newer ones just simply "add hearts" when you add health, and many of the older ones just change your "internal health" without doing anything to the amount of actual hearts you have.

Rather than be one of these plugins, you could do both. For each permissions node, you can have TWO options, not just one, a little like so:

PlayerHealth: (Integer, e.g. "40", meaning the player has a maximum of forty points of health)
HeartsDisplayed: (Integer, e.g. "10", meaning 10 hearts will be displayed)

The reason behind this, is that plugins which don't change the amount of hearts at all are, let's face it, lame. Usually because it's hard to do quick mental math in your head to figure out how much health you actually lost, as opposed to the amount of hearts you lost, which is potentially misleading.
And as for plugins that add hearts when changing health, there's simply a point at which there are too many hearts - in other words, uncountable. Rendering the whole simplistic "health is based on hearts" system useless.

An example scenario where this can be used is where all mob health (including players) is multiplied by 8 to 11 (depending on the mob), to allow for greater flexibility for differentiating damage in how much each mob/tool/projectile/whatever does. You could have your players suffer through seeing 40-70+ hearts, or you could leave your players with 10 hearts, wondering how much damage they just took from that last skeleton arrow.