World of Warcraft Addons

User Interface Resources 

Popular places to find addons

WorldOfWar.net

WorldOfWar.net maintains a great database of AddOns. They do a good job of weeding out the old and broken AddOns. They also do a great job of letting the author of the AddOn maintain his creation in the form of versioning, change notes, etc.

Curse Gaming

Curse Gaming is, I believe, a guild first and a UI site second. However, their UI database is gargantuan. This is both good and bad. The good is that you can find almost anything here. The bad is that old and broken AddOns are still in the database. I highly recommend paying attention to the dates of the AddOns before downloading.

Current Favorites 

ChatSound

This small addon will monitor ALL chat windows and look for a specified text phrase. If that phrase is listed in the ChatSound_config.lua file it will play the .wav file specified.

Atlas

Atlas provides a simple in-game instance map browser. If the Auto Select option is enabled Atlas will automatically open to the map of the instance you're in. Maps for every instance are included.

EquipCompare

In WoW when you shop for items at a merchant and you hover over an item, you get a comparison tooltip showing the "currently equipped" item too. This AddOn adds such a feature everywhere in the game where you can hover over items, such as in your bags, in the loot window or on the reward page of a quest. You also receive comparison tooltips when clicking an item link in the chat box.

Advanced Bags Plus

Advanced Bags is a UI Addon to help players manage all their items. It can replace Blizzard's bag system and can also work along side it. It provides a very easy to use UI to configure and manage how your items are sorted. Although it does organize and sort your items, Advanced Bags does not actually manipulate your item's location in your bags.

Mobile Frames

Makes selected frames mobile by dragging their top/title bar.

Confab

Confab allows the user to do pretty much anything they wish to the chat editbox. It comes with 4 basic styles as well as the ability to undock, move, resize (up to 1000 pixels and down to 300 pixels), lock position and turn off the autohide feature. It also allows the enabling of the cursor keys so you can use them to edit text as well as scroll through your history (instead of using ALT+arrows).

BGBuddy 2+

BGBuddy is a simple addon which tracks your progress in the battlegrounds and provides helpful features such as Auto-Join, Auto-Resurrect and Auto-Release.

Clean MiniMap

This mod provides total control over your Minimap including moving it and resizing it, hiding the titlebar, moving and hiding the icons around the map, and adding the three other compass points (S,E and W).

This mod allows you to zoom the minimap using the mouse wheel and to affect the opacity of the minimap by holding down the (configurable) 'modifier' key (default SHIFT) and scrolling the mouse wheel.

You can also have the minimap toggle between large, normal, and small sizes with a single keypress.

Discord Unit Frames

Discord Unit Frames lets you create the self, party, pet, and target unit frames you've always wanted. Each frame is divided into many separate, individually configurable elements that you can tweak to your desire. There are 10 boxes for text in which you can place variables to display just about any info on a unit that the UI allows in addition to the standard health bar, mana bar, icons, and portrait. Each can be moved while still remaining attached to its base frame, resized, recolored, made translucent, set to show on mouseover, and a lot more.

Discord Action Bars

Discord Action Bars hides the default interface art and provides you with access to all 120 action buttons to configure however you like. You can also configure the pet action bar, shapeshift bar, bag bar, and the micro menu buttons. The full description of this mod is huge... go check it out.

Discord Art

This mod provides you with 20 or more textures to configure and add to your UI. In addition to sprucing up your UI with artwork, they're also fully programmable buttons that can be set to do anything you can code in lua. Like all Discord mods, this one goes way beyond description. Check out what users have done on the Discord forums.

Group Buttons (Discord)

Group Buttons would be better labelled as Discord Caster Assist or some similar name. What it does is provide you with rows of action buttons attached to you, your party members, your pet, your party members' pets, raid members, one for hostile targets, and a separate one for friendly targets. Group Buttons is mainly intended for healers though any class can find uses for it.