WowAce

Splut

PreferredItems

Uses guild notes to add alts' main names to chat windows and other places

Simple oRA3 Maintank Target Frames, based on oUF

Toggles player names.

CraftList

Speeds up triggering actions such as items or spells by triggering them on key/mouse down rather than release

NabuFiveSec

BigTrouble

Helps you automate selling stuff

Allows you to prevent addons from sending and receiving messages.

Keeps track of durability and pops up a dialog to repair when you go to a vendor who can.

Tracks whispers and conversations

Adds options to create customized layouts from within grid.

Database of FlightMaster locations for HandyNotes

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

DeclineDuel

A simple range indicator for optimizing Halo damage.

Fires events whenever a raid member "fails".

Does a /who search of multiple dungeons.