This is a plugin for Details! addon, if you still doesn't have Details! installed, download through this link [[http://www.curse.com/addons/wow/details|click here]] or curse client.
This plugin tracks your current target and show in the window who is doing damage to it.
The amount of damage shown is the amount dealt since you selected the target.
The goal of this plugin is tell for a Target Caller in a RBG or a Raid Leader in a raid who is targeting and damaging the right target.