WowAce

ItemPriceTooltip

Keeps account of attendance and loot for raids, parties, bgs, arenas and solo for questing.

Easily mail friends, guild members, alts and others

Animated target indicator

Adds your damage to Scrolling Combat Text

HUD that you can tweak the way you want.

Keeps track of durability and pops up a dialog to repair when you go to a vendor who can.

Estimated time until current target will die.

Hemlock

New/Old BG indicator, Join by ID, Quick Leave Queue, etc.

Reduces interface tedium by doing the little things for you.

Incubator

Collect data about you myth+ challenges

Rebuff yourself with the mousewheel.

A library to provide a clean dropdown menu interface.

Tracks SendAddonMessage Spam

Tracks skill and tradeskill costs and progressions, as well as trainer locations.

Klappa

Database of FlightMaster locations for HandyNotes

Insanity resource tracker for shadow priests.