Brief description: "Theorycraft places everything you need to know about your spells right on their tooltips. It also has *some* melee and ranged support.
For casters, it's the ultimate in game tool for comparing your different gear sets. 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.
Theorycraft also has features that allow you to try different talent builds and gear configurations, and see how they'll affect your spells. (casters only currently). In 1.04 (testing at the moment) it'll have a ui interface for trying on the dungeon 1, 2 and tier 1 and 2 gear sets to your current gear. Just click the button, have a look at your new stats and hold your cursor over a spell to see how much more/less damage you would do in that set. Talents can be tested in a similar way.
Every time you move the cursor over a spell it parses your equipped items, your buffs, and the targets debuffs, to populate the tooltip with information. How much information is displayed is customizable through checkboxes by typing in /tc."
At the moment it is a pretty bad resource hog if you hover over tooltips in too little a time, not sure how much this mod could be scaled down but the original concept is what appealed to me, especially as a warlock and being able to calculate how much of a DPS boost new items will give you.