WowAce

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

Adds item values to chat messages.

T_ItemInfo

Sheppard

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

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

Automatically sells low quality gray items and selected white items to the vendor.

Quest information

FuBar_KTMFu

Adds Friend and Guild options to the context menu of player units.

A simplistic clock.

NCHide

Light-weight enhanced casting bars

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

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

Whats

Does stuff for sachmo

One-click smart mounting.

EQCompare

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