I find these addons really useful.
AI Research Grid: This shows the traits researched on your currently logged in character and your alts. It also shows styles learned for each character.
ESO Master Recipe List: You can access the entire recipe list (including unknown recipes and your alts) anywhere. It highlights the recipe needed for a writ in the provisioning screen and you can also track recipes.
Research Assistant: This adds a simple icon in the inventory view to show whether or not you have researched a particular trait.
Awesome Guild Store: This improves the Auction House window immeasurably. Search with either text and/or filters. Category filters for both your inventory and the store.
Master Merchant: This provides a ton of information about your sales and items currently on sale. It makes buying, selling and tracking sales so much easier.