WowAce

Am I locked out? - Displays all RaidIDs for all chars

Tracks the HoTs you cast.

Auto repair + shows durability.

Shows skills in a tooltip and allow them to be used. Also keeps track of Cooldowns.

Sells all junk in your inventory at once.

OneBag is a replacement for the default game bags that combines all the bags into one frame.

Bayi's Extended Questlog

Questing minion

A library to provide a markup syntax

Archaeology Assistant

A fancy LDB reputation display, show faction reputation and reputation change history

Minimap addon of awesomeness. *chewing sound*. It'll nibble your hay pellets.

Display auras on action buttons.

An extension to the default WoW Unit Frames.

Alternate Power- and Combopoint/SoulShard/HolyPower-bar

Unit frame addon using Ace3.

Max Camera

Show buffs on nameplates.

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

Niagara