WowAce

Give whispers an instant messenger feel.

tekKonfig

A library to handle rearrangement of blizzard's frames when bars are added to the sides of the screen.

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

A library to help with localization of talent trees.

Shows the possible loot from the bosses

Adirelle's bag addon.

Remember where rares were

Tracks rare drops, including companions and mounts

Adds predicted lists for spells while the user types

Modular damage meter.

"Debug" code free since '93!

Modular casting bar.

A skinning engine for button-based add-ons.

Test whether the player can really dispell a buff or debuff, given its talents.

Displays additional information for Mythic Keystone dungeons

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

Postal: Enhanced Mailbox support

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

Uses various methods to scan your surroundings for rare or specified NPCs.