WowAce

Automagical macros for each talent tier.

SRA

Tracking and ranking of dps/hps in a guild.

Gives detailed descriptions of particular localities, as zones or instances, and provides optional TomTom integration.

Display bag usage on each of your bag slots.

Display an informational fighter jet style HUD while flying or riding

Libary for cleanly inspecting other players

Displays the time remaining before your (temporary) pet is dismissed.

A Data Broker display for combat statistics.

mHud

Scans your Equipment for cumulative item bonuses and sums them up.

Helps you automate selling stuff.

EQCompare

A library for PvP and Battlegrounds

Enables user defined sounds for Party, Guild, Raid and Custom Channel chat messages.

Aspect checker addon for Hunters.

Niagara

CCBreaker

BuffMe

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