WowAce

Allows you to create icons, which can be customized greatly by user's input. CiR is more than just the usual cooldown tracker.

MonkeyPirate

Tells you who bought your auctions.

What actual item level is an item? Yes, it's horrible we need a library for this.

Warnings about tanking specific abilities

FelwoodFarmer

Create reagents easily

Keeps track of time played and rested XP across your characters.

Automatic tracking of the current target's creature type. For the discerning PvE hunter.

Capital Cities NPC Browser

Test whether the player can really dispell a buff or debuff, given its talents.

Cryolysis

Simple language selection for Data Broker

SpyGnome

FuBar_KTMFu

Color manipulation library.

Helps you keep track of your experience.

Displays an animated bar with next enemy swings

DogChow

Allows the user to customize the the tooltip with lua-scripts