WowAce

Personal buff monitor - are you buff enough?

FuBar_BattlegroundFu

Visualises healing.

Litmus

Displays information about the current zone on the World Map

Mutes various chat related annoyances while in capital cities and instances.

Adds unreal tournament style sounds to PvP encounters.

Manufac

Gathers statistics on certain chance-on-use spells, also known as procs.

Open all your mail

Add item levels to Inventorian

Mutt (Macro Update Target Tool) provides slash-commands that can patch the predefined targets in your macros.

PT3Browser

Adds predicted lists for spells while the user types

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

KillingSpree

Show what quests have changed

Adds your name to chat messages

MorgDKP

Priority 3 loot announce addon