First one out is Arkinventory.
This is a bag addon that's simply just a dream to use.
At first sight after installing it, it looks like just another normal addon that shows all bagspace in one window, but once you start setting it up, there are many ways of improving on that simple feature.
Starting off with setting up different categories, such as Blacksmithing items, soulbound armor, junk, food, flasks and so on, you can already see that it sorts whatever is in your bags to whatever category you set the items to go to.
While this may seem trivial, as many prefer manually sorting their bags by themselves, the functionality of Arkinventory quickly makes you forget that orderly fashion you're used to. It simply does that for you.
But there is more than just the default category. And I'm not talking about the ability to add more slots to add categories to, but the "Ruleset" options. A full list of them can be found here.
Let us say you're like me; you got 4 or more itemsets in your bank and bags. That's about 17 items you have to switch around and keep control of where is for each itemset.
By making a rule, you can now sort the items based on what you've decided to call the itemsets in outfitter, blizzard's own gearchange option or other addons, such as wardrobe and the likes.
Hard to do?
Not really. It's simply to look up what you're trying to do on the wiki, then use one of the examples listed.
For instance, I got a set for tanking, named Tank, a set for damage, named Retri and a set for healing, named Heal. (I know, fancy names, go me.)
To sort these sets, I must make 3 individual rules with the text "outfit("setname")". So for me, I need to make:
Trust me, this saves you a -lot- of hassle when you're sorting out your gear when you're looking at a potential upgrade, by not having to mouseover every soulbound item in your bags.
There are, of course, a lot of other rules you can set, but that's for you to figure out which ones you have use for.
This is the first addon I download whenever I've formatted my harddrives.