WowAce

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

Co-ordinated targetless Party and Raid wipe recovery. Evolved

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

Insanity resource tracker for shadow priests.

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

A customizable casting bar replacement for TBC Classic & Classic WoW

Out of Mana bar.

Buff reminders and more

Mana regen rate display.

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

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

Displays your soulshards in better way.

ControlFreak

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