These AddOns are currently the Favourite WoW AddOns.
(1) Atlas
Atlas is a simple instance map browser. If the Auto Select option is enabled Atlas will automatically open to the map of the instance you are in. You can find Maps for every instance.
(2) EquipCompare
One of the favourite wow addons - gives you a comparison tooltip showing the "currently equipped" item when you shop for items at a merchant and you hover over an item, You can find 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. By clicking an item link in the chat box, you also receive comparison tooltips.
(3) ChatSound
Although it is a small addon, it is one of the favourite wow addons. It monitors 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.
(4) Mobile Frames
One of the favorite wow addons. It makes selected frames mobile by dragging their top/title bar.
(5) Advanced Bags Plus
UI Addon that helps players to manage all their items. Use it to configure and manage how your items are sorted. It either replace Blizzard's bag system or it work along side it.
(6) BGBuddy 2+
It is a simple addon which tracks your progress in the battlegrounds and provides helpful features such as Auto-Join, Auto-Resurrect and Auto-Release.
(7) Confab
It comes with 4 basic styles. It has 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). Confab allows the user to do pretty much anything they wish to the chat editbox.
(8) Clean MiniMap
One of the favourite wow addons. It gives you total control over your Minimap including moving it and resizing it, hiding the titlebar, moving and hiding the icons, and adding the three other compass points (S,E and W).
This mod allows you to zoom the minimap using the mouse wheel, 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.
(9) Discord Unit Frames
Use this addon to 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.
(10) Discord Action Bars
Use the Discord Action Bars to hide the default interface art so that you can 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.
Good place to find Current Favorite WoW 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.