WowAce

Quest History tracker and shows lists of quests in each category

Reports in raid chat whenever a raid member "fails." /EnsidiaFails to configure.

Automated smart raid marking to speed up trash clearing in instance runs

Co-ordinated targetless Party and Raid wipe recovery. Evolved

Adds World Quests to the quest list

Shows simple Buttons for your Cooldowns :)

Queing of /who and SendWho() queries and a much better interface (see docs), with gurantee to be executed & callback.

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

Displays online guild mates.

Show where the chests are for Withered Army Training

manipulate the minimap in numerous ways; move, scale, skin.

A replacement tradeskill window

Library providing mappings between trade skill recipes, spells and created items

Database of Vendor locations for HandyNotes

Allows the user to preview and track their progress in the collection of various transmogrification appearance sets

MorgDKP

Raid Frames using the oUF framework.

A simple slash-command-based addon for querying item information from the Blizzard servers via ItemIDs and SpellIDs.

an embeddable library providing an abstraction layer for tracking and querying Blizzard's Nameplate frames with ease and effi...

Tracks the HoTs you cast.