WowAce

Library providing multi-column tooltips.

Scan for rare mobs so you can find them and their loot

Show the appearance of an item when you mouse over it

Set and manage character notes that are displayed in unit tooltips and /who results.

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

Lightweight but powerful auction house interface

Item stat breakdown, analysis and comparison

Collectinator is an addon which will scan your pets and mounts and provide information on how to obtain them.

Complete tradeskill information.

Simple and super light-weight tracking of profession links for all your alts.

Kaliban's Class Loot List Star Display

Displays item durability and repair cost.

Adds various information based on PeriodicTable to item tooltips.

Shows where the equipment of inspected users or yourself is from.

Tooltip of awesomeness. Moo. It'll graze your pasture.

Manages social information with a focus on main-alt associations.

StarTip (Tooltips from Outerspace)

Shows where the equipment of inspected users or yourself is from.

Add-on that shows in a unit tooltip window a number of killed mobs of the same kind

Shows mana usage as a percentage in a spells tooltip.