WowAce

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

A customizable casting bar replacement for Classic WoW

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

Co-ordinated targetless Party and Raid wipe recovery. Evolved

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

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

Buff reminders and more

ControlFreak

Displays your soulshards in better way.

Mana regen rate display.

Out of Mana bar.

Displays a graph of the player's Mana regeneration

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

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.