This is Sephyx's TheoryCraft 2. What does it do? Here's a description straight from the mod author:

"Theorycraft places everything you need to know about your spells right on their tooltips. Or, if you're using the default action bars, on their buttons. Want to have a row of low rank heals with their heal values on the button? Too easy.

"Not only that, TC can let you compare any gear set you wish. Curious as to how much that crimson felt hat actually increases your dps by? It'll tell you that. Or how much 1 extra crit is worth in spell damage. Or for melee users, how much attack power you'd need to equal another 10 agility, or 1% to crit. Or perhaps just how much damage you would do were you lucky enough to land a Hammer of Bestial Fury or Ashkandi."