DrDamage
In-game theorycraft. Calculates actual damage/healing based on gear, talents and buffs.
Cataclysm 
The Burning Crusade 
Wrath of the Lich King Addon Details
Expansion
Developers
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DrDamage displays the calculated, average damage or healing of abilities with talents, gear and buffs included on your actionbar buttons. The addon will also add various information to the default tooltips in your spellbook and on the actionbar.
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