WowAce

Collects, analyzes and presents information about the virtual economy.

Creates simple table controls for tabular text data.

Forces LSM and LibSink to use specific textures and sinks.

JasonQuest

Open inventory items with 1 button

pDebuffList

FuBar_VolumeFu

Volume control panel.

Provides database of quest giving npcs/mobs/objects for display on the World Map/Minimap

Display the global cooldown around your cursor. (+ other stuff)

NextStep

Shared handling of media data (fonts, sounds, textures, ...) between addons.

Easily mail friends, guild members, alts and others

Pet Theory

Smooth group roll frames

Database of important locations for HandyNotes

Quixote

Provides the /in command for delayed execution.

Raid loot system.

Sniff