WowAce

Bulk mail sending made really easy

Queing of /who and SendWho() queries and a much better interface (see docs), with gurantee to be executed & callback.

Shows you a much bigger icon selection frame

Collects Herbs, Mines, Gas Clouds and Fishing locations and adds them to the worldmap and minimap

GrindStatus

Keeps track of HoTs, DoTs, Buffs and Cooldowns. With audio and text alerts to refresh them.

Manufac

Shows simple Buttons for your Cooldowns :)

Bayi's Extended Questlog

Watches your chat, flagging missplled words, and offering correctly spelled suggestions.

Raid Frames using the oUF framework.

Adds countdown voices from Heroes of the Storm to BigWigs for all supported locales.

Checks your gear before bosses and yells at you if you're an idiot

Create panels for customizing your UI, sucessor to eePanels

Track all threat redirection ability transfers and cooldowns (Misdirection and Tricks of the Trade).

Regain that wasted space!

A blizzard group finder replacement

Quest History tracker and shows lists of quests in each category

FlexBar2

Configurable Castbars