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

Displays your soulshards in better way.

Buff reminders and more

ControlFreak

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

Experiments in Mana/Regen Tracking

Out of Mana bar.

Displays a graph of the player's Mana regeneration

Mana regen rate display.

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

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