WowAce

Adds predicted lists for spells while the user types

Keeps track of durability and pops up a dialog to repair when you go to a vendor who can.

A library to provide a clean dropdown menu interface.

Detects specific CDs and spellcasts

Shows mana usage as a percentage in a spells tooltip.

Klappa

Tracks SendAddonMessage Spam

Tracks vulnerability debuffs on multiple mobs

Displays online guild mates.

This will keep track of how much you have paid per item in your inventory. Only one of (Auctionator, Auctioneer, AuctionLite,...

Quest library for mob location and other information

HiveMind

Shows your standings with the factions.

BiGOpt

Display an informational fighter jet style HUD while flying or riding

Auto roll greed, need, pass or manually roll on items.

FuBar_FarmerFu

Tracks skill and tradeskill costs and progressions, as well as trainer locations.

Review any raid or party members recent incoming combat along with health level.

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