WowAce

Adds the toon name to the alert that your RealID friend has come online.

Database of Diminishing return categories and general data

Better quest log and tracker

TradeStop

Suppresses chat messages from damage meters and provides single chat-link damage statistics in a popup.

A Simple World of Warcraft Spam Filter!

Broken in Midnight. I recommend MidnightFocusInterrupt instead.

Displays classes and specialisation dependent stat priorities provided by icy-veins.com

FuBar_ChatLockFu

Shows Status Bars that you can drag around and group up for your cooldowns

Displays the locations of Garrison music scrolls.

Expo

LDB data source for Tradeskill Cooldowns

An addon to assist with organizing healing in a raid set up.

A library that handles the basics of "window" style frames

Dropdown library for Ace3 options, extensible to other options formats. Works ok.

Open all your mail

TouristLib

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

Separate indicator for your HoTs with timer.