WowAce

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

Show buffs on nameplates.

Checks your gear before bosses and yells at you if you're an idiot

GrindStatus

Collects Herbs, Mines, Gas Clouds and Fishing locations and adds them to the worldmap and minimap

Manufac

Keeps track of HoTs, DoTs, Buffs and Cooldowns. With audio and text alerts to refresh them.

Create panels for customizing your UI, sucessor to eePanels

Configurable Castbars

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

XRS

Quest History tracker and shows lists of quests in each category

CandyBar style cooldown display

Displays the remaing cooldown of the action you tried to use

GotWood

Track all threat redirection ability transfers and cooldowns (Misdirection and Tricks of the Trade).

Enables multi-line/unlimited-length pasting of text or commands into WoW

Redirect character errors, system messages and information messages.

Regain that wasted space!

Watches your chat, flagging missplled words, and offering correctly spelled suggestions.