WowAce

Display aura information inside action buttons

StatLogicLib

Drop the cheapest thing you're carrying

Keeps track of your top hits, heals, criticals, etc.

Keeps track of your current location.

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

Displays Instance Maps

Configurable Castbars

tracks and share quests between your party

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

Raid Frames using the oUF framework.

DKP system for all guilds in the Banana Republic.

Create panels for customizing your UI, sucessor to eePanels

Alternate Power- and Combopoint/SoulShard/HolyPower-bar

Displays Pet Info for Hunters and Warlocks. Also keeps track of Stables and Pet and Demon skills.

A blizzard group finder replacement

Co-ordinated targetless Party and Raid wipe recovery. Evolved

FuBar_TrainerFu

Try on different gear and see their total stats and their differences. In short, an ingame CT-Profiles-like addon.

Track all threat redirection ability transfers and cooldowns (Misdirection and Tricks of the Trade).