WowAce

Detox

Find five Ephemeral Crystals, get a pink hippogryph

View an image of your guild vault from anywhere.

Co-ordinated targetless Party and Raid wipe recovery. Evolved

FuBar_TrainerFu

Displays online guild mates.

Death analysis tool

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

Display aura information inside action buttons

Wowhead Data dump for GatherMate

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

View bank, inventory, mail, equipped and guild bank contents from any character anywhere!

Shows simple Buttons for your Cooldowns :)

FuBar_PTReagentFu

Checks your gear before bosses and yells at you if you're an idiot

Quest History tracker and shows lists of quests in each category

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

Incoming Heals

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

Database of Vendor locations for HandyNotes