WowAce

Calculates incoming damage avoidance and mitigation

Battleground Timers in a HUD around the Blizzard BG status

A library to get currencies' related information

Visual Raid Cooldown Tracking

VitalWatch

Open those pesky lockboxes

FuBar_VampwatchFu

Uses guild notes to add alts' main names to chat windows and other places

GemCensus inspects people and records which gems they have socketed. Then displays which gems are the most popular.

Square Minimap and more

Saves contents of your guild bank and sends/receives updates over the addon channel

Auto sizes frame layout depending on group/raid size or predefined zone data

ImprovedTargetFrame

DHUD2

Libary for cleanly inspecting other players

FuBar_AlchemyFu

Persists collapsed state of headers across wow sessions.

DaHud

Niagara

Speeds up triggering actions such as items or spells by triggering them on key/mouse down rather than release