WowAce

In-game theorycraft. Calculates actual damage/healing based on gear, talents and buffs.

Target Debuff monitor. Stacking aware, giving current and potential debuff status.

Co-ordinated targetless Party and Raid wipe recovery. Evolved

Insanity resource tracker for shadow priests.

A customizable casting bar replacement for TBC Classic & Classic WoW

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

Buff reminders and more

A simple visual combat log with spell/skill icons for Classic WoW

Out of Mana bar.

ControlFreak

Displays your soulshards in better way.

End of combat reporting for feedback on how your cast time was spent or wasted.

Mana regen rate display.

Experiments in Mana/Regen Tracking

Manaburn

FuBar_MageFu

Simple addon that tracks internal cooldowns.

Templar is the ultimate Mage Assistant

Addon for knowing exactly when you get out of combat so that you can drink.

OrbMaster