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

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

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

Item stat breakdown, analysis and comparison

Lightweight but powerful auction house interface

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

Complete tradeskill information.

Adds various information based on PeriodicTable to item tooltips.

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

StarTip (Tooltips from Outerspace)

Kaliban's Class Loot List Star Display

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

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

Displays item durability and repair cost.

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

Shows mana usage as a percentage in a spells tooltip.

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

Provides shareable notes and other information for your guild, party and raid community.