WowAce

Keeps track of group members and keeps an up-to-date cache of their specialization and talents.

"Debug" code free since '93!

Arena unit frames

Simple AceGUI3 dropdown item classes for font/statusbar/sound/etc.

Replacement for the default UI's StaticPopup dialogs.

Database of item upgrade IDs

Tracks Raid/Instance/World Boss lockouts for all your characters, daily/weekly quests, currencies, trade cooldowns, and more!

The original source for where things are in Pandaria

Parses ToC and adds About panel to Interface options

Allows you to view all items from all your characters on the same account, when-ever and where-ever you want.

Estimated range display. Type /rangedisplay for config

Another HUD addon

In-game theorycraft. Calculates actual damage/healing based on gear, talents and buffs.

Shows the possible loot from the bosses

Simple and super light-weight tracking of profession links for all your alts.

Modular damage meter.

Simple Map Mod

Officially the original source of knowledge of where things are in Draenor during Warlords, in HandyNotes

A skinning engine for button-based add-ons.

Library to check whether an item is equipable / appropriate for a given class