MeSmash!

This addon will compare your baseline stats to your current stats and display it as percentage. Intended to show how valuable your current short-term buffs like potions, trinket procs and heroism are.
When number is low, save your resources. When it is high, spend them.

To save baseline stats, either click "Set current stats as baseline" in options or right click on frame while holding left ALT.

Type "/mesmash" to open options.

Compares your primary stat (spell damage/attack power/ranged attack power), haste, crit and mastery.