DamageMeters
http://www.curse-gaming.com/en/wow/addons-791-1-damagemeters.html
Okay, thisone's strictly for fun... and gloating. DamageMeters collects all the combat data from nearby characters and compares them in a color-coded bar graph that enables you to mouth off with pride when you're topping the damage-dealing charts. Of course, it also lets you silence annoyingly vocal members of the group that dish out a lot of damage, but also need to be healed more than anyone else (yup, it tracks the amount of healing everyone receives as well). The best part is that you can have DamageMeters report its statistics at any time into any chat window you choose, so you can instantly back up your pompous proclamations with hard numbers. What could be better than shaming everyone around you with your obvious (and provable) superiority?
