WowAce

Resizes some default UI frames (GuildUI, LootFrame, RaidInfo, TradeSkillUI and TrainerUI) to show more lines

Mapster's Minimap brother

Embeddable chat throttling library with traffic shaping and priorities

Enables the filtering of items when you visit a vendor

Simple display and management of Away / AFK / Busy / DND status via broker or minimap

Automatically and immediately cancels all in-game cinematics and movies

Angry Raid Points Client

SurgeonGeneral

Display the global cooldown around your cursor. (+ other stuff)

A collection of useful (guild management) tools and utilities for smarter than average bears.

AutoMagicly clicks though Daily Quests, and Turns them In.

Customizing your buff display

Pet Theory

FuBar_UIReloadFu

A filter tool for blizzards TradeSkillUI

Some handy events that should exist in the default UI

Announces interrupts in the chat

LDB display of currencies, gold totals and session / daily / weekly statistics for all characters on a server.

Constructs a SimC export string for an item without equipping it

Allows for easy congratulating when someone earns an achievement.