WowAce

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

Manages the scanning of the auction house in a reliable an efficient manner.

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

MiniPet, Always keep your little companion with you, based on the work of Carra.

FuBar_KTMFu

Helps to track outgoing mails

TableTopLib

Open those pesky lockboxes

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...

Handles the assignment, coordination, and announcement of many parts of the Lich King encounter in Icecrown Citadel.

Adds item values to chat messages.

ClassColors

MobHealth

Quest information

opens/closes all bags when at merchant (instead of only the backpack)

SanityItemCache

Simple, nice-looking power bar addon for all classes.

NCHide

Open those clams (and other containers)

One-click smart mounting.