WowAce

An auction house AddOn to make the life of each fence easier.

Keeps track of your reagents for buffing.

Keeps track of space left in your bags.

Auto and random summon highly customizable for your pets and mounts, with Data Broker support.

Prat

A library for PvP and Battlegrounds

Shows simple Buttons for your Cooldowns :)

Helper functions used by my addons, feel free to use them in your addon too

WhoDrop

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

Achievements

Provides shareable notes and other information for your guild, party and raid community.

FuBar_VampwatchFu

FuBar_HonorFu

ItemSync the happy item gatherer and toolkit.

FuBar_ExperienceFu

A library to help with localization of item types and subtypes.

Tells you if you're buffed

A simple slash-command-based addon for querying item information from the Blizzard servers via ItemIDs and SpellIDs.

Shows Status Bars that you can drag around and group up for your cooldowns