Did you ever loot stuff and didn't know whether it was better than the one you have on your bank for your second role? Yeah, that's bad.
With this addon you can always take a look at any equip you want. Just take it on and add it as a new set. If something interesting drops, just take a look at your saved set and compare the values.
After installation you can open and/or pin it with the start-menu-like button or by typing /afalt in any chat window.
What does it not do?
Sadly it's not possible to save the rune slots, too, so you can just compare the base items.