The premiere pet class, Hunters tame beasts of Azeroth to do their bidding while utilizing gun and bows to fell their foe. They excel at gaining the killing blow in player versus player combat, more of which can be read about in Saris' PVP Guide for Hunters. There are many different ways to play a hunter, but all of them use mana, which is why Viper Notify is the Hunter's addon of choice.
While it doesn't provide extra bars or a plethora of cooldown information, and it lacks a simple user interface, Viper Notify is one of the single finest Hunter addons available. Simply stated, Viper Notify will let the player know when their mana reaches a certain threshold, letting them know to switch from Aspect of the Viper, which regens mana quickly at the sacrifice of damage, to their preferred fighting aspect. A simple tool, but an invaluable one.