WowAce

Provides a quick display for the LFG Call to Arms

Adds a button to the quest completion interface which allows you to auto equips selected quest Item after completion

ItemPriceTooltip

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

Bind spells, items, and macros to keys, without using action buttons

XArt

Better quest log and tracker

AspectsBar

AutoMagicly clicks though Daily Quests, and Turns them In.

Heads Up Display with Player and Target Info

Tells you the Tiller, what food they like, and if you have fed them today for the quests A Dish for ...

Display bag usage on each of your bag slots.

Angry Raid Points Client

Tracks SendAddonMessage Spam

Announces interrupts in the chat

Open inventory items with 1 button

Threat-2.0

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

HUD that you can tweak the way you want.

LDB display of currencies, gold totals and session / daily / weekly statistics for all characters on a server.