Bukkit

There are three different types of health bars: after a player's tag over his head, under the tag, over mobs' heads.

Health before and after the tag

Bar mode

There are 6 styles for the players' health bars: you can set in the config a number between 1 and 6. Remember to set this in the configs:

player-bars:
  enable: true
  text-mode false
  display-style: (see below)
  below-name: false
  # Only if you don't want the health even under the name

Text mode

At the moment text mode is not configurable. It will show like this:

Under the tag

The health is displayed under the player's name. The text you set in configs will be after the health value and can't be unique per player (cannot be changed due to minecraft limitations). You can use formatting codes (&4, &l, &n, &k) and a placeholder ->   <3   that will be replaced by a heart symbol (❤). Remember to set this in the configs:

player-bars:
  enable: false #if you want only the health under the name
  below-name: true
  below-name-text: (see examples below)

below-name-text: '&c<3'
below-name-text: '/ 20 &aHP'
below-name-text: '&0&oHealth'

On mobs

Bars mode

At the moment there are 4 different styles. Remember to set this in the configs:

mob-bars:
  enable: true
  display-style: (see below)
  text-mode: false

Text mode (default)

The health is displayed as text, remember to set this in the configs:

mob-bars:
  enable: true
  text-mode true
  custom-text-enable: false #custom text mode is explained in the next paragraph

It will appear like this:

Text mode (custom)

Here you can set the position of each element, set this in the configs:

mob-bars:
  enable: true
  text-mode true
  custom-text-enable: true
  custom-text: (see below)

Placeholders:

Some examples:

custom-text: '{name} &c{health} &4<3'
custom-text: '{health} &5&lHP'
custom-text: '{health}/{max}'
custom-text: '&6{health}/{max} &f- {name}'

Custom files

Here you can make your custom health bars! There are two similar files: one for players, and one for mobs. You have to set a bar for each stage, you can use color codes and formatting codes. There are some placeholders, because symbols cannot be stored in yaml files:
<3 = ❤
[x] = █
[/] = ▌
[*] = ★
[p] = ●
[+] = ♦
[++] = ✦

Note: you cannot use placeholders like {name} and {health} here! Use the custom-text instead.
The files are made of 10 (for mobs) and 20 (for players) lines, each one represent the state of the bar at a certain percentage of health.

Example (mobs):

5-percent-bar: '&c5% HP'
10-percent-bar: '&c10% HP'
. . .

Example (players):

10-percent:
  prefix: '&c'
  suffix: ' 10% HP'
20-percent:
  prefix: '&c'
  suffix: ' 20% HP'
. . .

Using the color codes as prefix, you'll be able to format the name of a player.