WowAce

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

Co-ordinated targetless Party and Raid wipe recovery. Evolved

Displays your soulshards in better way.

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

Buff reminders and more

ControlFreak

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

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

Out of Mana bar.

Mana regen rate display.

Experiments in Mana/Regen Tracking

Displays a graph of the player's Mana regeneration

OrbMaster

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

Templar is the ultimate Mage Assistant

Simple addon that tracks internal cooldowns.

FuBar_MageFu

Manaburn