Acki's Recipe List aka ARL is a very handy tool that was introduced before the 'Unlearned' tab was implemented in to the default profession window. It identifies patterns and recipes that you character does not know then allows for filtering by location, acquisition, expansion and much more. It's a modular system so you only need to install the corresponding profession module that you are interested in.
Another recipe related addon, this one will attach an additional tooltip to any pattern or recipe that creates an item, the extra tooltip will be of the item that is crafted which would then also contain information or data from other addons like TSM or TUJ pricing data. This helps in decision making on purchasing recipes, or identifying if you have the recipe on another character when you can see crafting costs from TSM.
This one is a bit more well-known however is very useful for quickly identifying active World Quests in Broken Isles and Argus to help you decide which one's to complete. Easily seeing the
Sangue di Sargeras quests or any work orders that provide the Rank 3 recipe you've been hunting for saves a lot of time!
NomiCakes displays how many recipes are learnable from each ingredient on Nomi's work order options, and how many of that ingredient you have in your inventory. This stops any wasted ingredients and helps you focus in on any missing recipes you might be fishing for. I personally would have named this addon
"NomiSteaks", since afterall Nomi is the best chef in Azeroth.
Filtering out your servers Trade channel is important to be able to see the actual trading going on,
as /u/Vhaea points out. BadBoy will hide all of the RMT spam, guild recruitment,
Furiatonante, Lama Benedetta del Vento Eterno spam, and anything else you want to filter by keyword.
AuctionValue is a super simple addon that adds your total Auction Buyout to the Auctions tab of your Auction House:
Another super simple addon, this one will increase the size of your loot window to display more items without having to click through the different pages.
Inventory management is vital to a gold-maker. Bagnon is the one-giant-bag conversion, which recently received an update that supports patch 7.3. ArkInventory is another popular choice for TSM users, your groups can be used as inventory categories for very easy filtering and management in your bags.
Despite recommending an addon to remove spam, I am also recommending an addon to handle sending periodic messages to your Trade channel. If you're regularly buying trade materials in bulk or are hunting for particular items you can use this addon to post a message about it. Please keep in mind the frequency at which the message is posted, don't go overboard!
FasterLoot is a newer addon among the many that try to speed up your looting. However this one is designed with other addons that use the loot window in mind, such as LootAppraiser and TSM Destroying. Sometimes you don't want to loot as fast as possible, just a little bit faster.
GatherMate2,
GatherMate2_Data and
FarmHUD are all excellent addons to help improve your farming efficieny. Another addon to include in your farming toolkit is
FarmIt, this adds an actionbar-like overlay to track specific items that you're farming so you don't need to keep checking your bags. It allows you to set goals or targets and counts down how many more of a particular item you need to finish your task.