WowAce

Open all your mail

Displays Instance Maps

Add-on that allows you to setup and quickly switch between different profiles of your action bars

KoS-Management Tool

Highly-configurable shield / absorb tracker.

Some handy events that should exist in the default UI

Checks your gear before bosses and yells at you if you're an idiot

Mutt (Macro Update Target Tool) provides slash-commands that can patch the predefined targets in your macros.

Display aura information inside action buttons

Smart raid loot and DKP tracker with automated standby and learning

Tracks, transmits and displays the cooldowns of your raid.

Cartographer_Treasure

View an image of your guild vault from anywhere.

Keeps track of performance of memory, latency, and framerate.

Adds your name to chat messages

Show what quests have changed

PT3Browser

Provides quick access to information on your group and their equipment, enchant and gem choices.

A Collection of Automations and Interface Tweaks.

Keeps track of current money and all your characters on one realm.