WowAce

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

Grabs bugs for the bug sack.

Library that handles chat output.

Toss those bugs inna sack.

Simple and Advanced combined - Bartender4 ActionBar AddOn

Replacement for the default UI's StaticPopup dialogs.

Postal: Enhanced Mailbox support

Parses ToC and adds About panel to Interface options

Collects Herbs, Mines, Gas Clouds, Archaeology, Treasure and Fishing locations and adds them to the worldmap and minimap

Scan for rare mobs so you can find them and their loot

Chat addon, filtering messages and UI changes

Displays additional information for Mythic Keystone dungeons

Makes the default frames moveable via simple drag and drop

Tracks rare drops, including companions and mounts

Replacement for ActionButton_Show/HideOverlayGlow APIs

An intuitive keybindings library.

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

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

Restores access to removed interface options in Legion

Modular casting bar.