It automatically alerts you whenever a rare mob is near. It tells you if there are no rares in the pet(s) you're battling and lets you hit 'Escape' to cancel the fight, rather than having to click the white flag and then click the confirmation dialog. Perfect for anyone seeking a specific breed of battle pet. It displays the locations of all pets on the world map, tracks all pets you have still to catch in a given zone, displays all your enemies and allies abilities as you fight, and it even suggests which is the most appropriate pet for your current enemy.
For help installing, see WoW Interface's FAQ. The World of Warcraft community has created a variety of top-notch mods for small pet enthusiasts.