WowAce

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

Insanity resource tracker for shadow priests.

Co-ordinated targetless Party and Raid wipe recovery. Evolved

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.

ControlFreak

Displays your soulshards in better way.

Buff reminders and more

Out of Mana bar.

Experiments in Mana/Regen Tracking

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

Displays a graph of the player's Mana regeneration

Mana regen rate display.

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