Silver Dragon
Unbelievable addon. It incorporates all the best methods of finding Rare mobs in any zone. I've found numerous rares since installing it.
It basically can scan the local cache for "sightings" of Rare mobs and will pop up a box telling you the name of the mob and the box is able to be used for targeting too.
The ldb plugin will show the possible rares in your current zone when you mouse over the button, with additional info showing when and how it was found with coordinates to boot.
Screenshot of the "Found a mob" box:
*EDIT: The bar showing all my plugins is Fortress. It's an LibDataBroker display