Hier einmal ein paar nützliche Addon's. Mit Erklärung. (Leider auf Englisch)
Ich werde diese liste immer wieder erweitern.

Addon Control Pannel
ACP adds the "Addons" button to the game's main menu (The one you get when you hit ESC). It allows you to manage your addons in game, with an interface which looks similar to the blizzard addon manager.

Atlasloot Enchanted
What is Atlasloot AtlasLoot Enhanced is an UI mod allowing for loot tables of bosses to be browsed whenever needed within the game.

Bagnon
Bagnon is an addon that merges all of your bags into three frames: One for your inventory, one for your bank, and one for your keyring. Beyond the basic all-in-one inventory functionality

Bartender 4
Bartender4 is a full ActionBar replacement mod. It provides you with all the features needed to fully customization most aspects of your action and related bars.

Clique
Clique is a simple GUI that lets you assign click-casting for any number of unit frames. Click casting allows you to define the behavior the game takes when you click on a frame. This can be casting spells, running a macro, using an item, or something as simple as changing targets or assisting a unit.

Deadly Boss Mods
Boss mods for all raid bosses

Decursive
Decursive is a cleansing mod intended to render affliction removal easy, effective and fun for all the classes having this ability.

Gladius
Gladius adds enemy unit frames to arenas for easier targeting and focusing. It is highly configurable and you can disable most features of this addon.

Grid
Grid is a modular, lightweight, and screen-estate saving grid of party/raid unit frames.