WardrobeAchiever is an addon for World of Warcraft that helps you easily track which items you have collected already to complete an appearance achievement. Right now in the game you can see if you have completed a specific appearance achievement or not, but you cannot tell which items you are missing in order to complete it. The only option you have is to look it up in the collections menu or in wowhead.
In addition there are several achievements that you cannot complete with specific characters (like newers classes that don't have older PVP/PVE sets) making it even harder to know which ones you can complete.
With WardrobeAchiever you will be able to see the list of achievements that you can complete with the current character, which achievement you already have and track which parts of the set you are missing (as you can do in the collections menu).
Currently this is a release candidate version, however I have to recheck with every class/facction that the tiers showed are the correct matches, which I will finish doing this week.