WowAce

Creates and manages secure Action Buttons

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

Another HUD addon

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

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

Modular casting bar.

A skinning engine for button-based add-ons.

Restores access to removed interface options in Legion

Postal: Enhanced Mailbox support

"Debug" code free since '93!

Modular damage meter.

A library allowing for easy creation of graphs

Adds a Button to the Worldmap which toggle Handynotes

Tracks rare drops, including companions and mounts

Wowhead Data dump for GatherMate2

Shared handling of alternate identity data between addons.

Shows the possible loot from the bosses

Library of compressed itemid sets.

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

Adds countdown voices from the heroes in Overwatch to BigWigs.