WowAce

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

A modular, lightweight and screen-estate saving grid of party/raid unit frames

Creates and manages secure Action Buttons

Tracks rare drops, including companions and mounts

A library to help with localization of in-game subzone names.

Shared handling of alternate identity data between addons.

Provides one or many frames for dumping text into.

Modular casting bar.

Arena unit frames

Show item levels in the character frame

A library to help with localization of the output of UnitCreatureType.

Simple Map Mod

Replacement for the default UI's StaticPopup dialogs.

"Debug" code free since '93!

Restores access to removed interface options in Legion

Officially the original source for where things are in the Broken Isles, in HandyNotes

A library to help with localization of races.

A skinning engine for button-based add-ons.

A library to help with localization of bosses.

Adirelle's bag addon.