WowAce

On login, displays changes to your guild since the last time you logged in.

Adds notes to item tooltips

Tracks Black Market Auction House auctions.

TableTopLib

A library to try to guess a level of a player.

RaidHelper

Shows skills in a tooltip and allow them to be used. Also keeps track of Cooldowns.

Addon management panel extreme release edition

Cartographer_Mailboxes

Adds an additional spell alert if you have more than one charge of a buff that is normally tracked by the in game spell alert...

Add TomTom Waypoints for Flight Masters in classic

Sheppard

Raid level alternative player power warning (Atramedes, Cho'gall, Lord Rhyolith)

Shows the price of a single item in the Auction House.

Shared handling of media data (fonts, sounds, textures, ...) between addons.

Lets you drag UI frames around

Welfare Epics

Provides the ability to suppress specific spellcasts like "Deaden" from your casting bars.

FuBar_KTMFu

Right-click on minimap blips to open a targeting menu (out of combat).