WowAce

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

Library providing multi-column tooltips.

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

StarTip (Tooltips from Outerspace)

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

Lightweight but powerful auction house interface

Item stat breakdown, analysis and comparison

Complete tradeskill information.

Adds various information based on PeriodicTable to item tooltips.

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

Show the appearance of an item when you mouse over it

Dynamic item DKP calculation

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

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

Healer gear benchmark

Shows mana usage as a percentage in a spells tooltip.

Kaliban's Class Loot List Star Display

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

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

Show extra information about Jewels and Enchants in tooltips.