WowAce

For Developers (of Addons which uses LibWho-2.0): show queues and queries in realtime

Provides access to all teleporting spells and items on right click of minimap button.

Shift-clicking on a user name in chat shows a tooltip with their whois information.

Audible Alerts for self-gain, self-lost, any-gain, all-lost of statuses.

GratuityLib

Records mail when visiting the mailbox to make keeping track of mail and expiration dates easy.

Replacement for the default minimap.

BigWigs_ThogarLaneColors

LibDataBroker plugin showing some random XP related data

Color manipulation library.

TrickOrTreat

FruityLoots

FuBar_TinyTipFu

Tracks specialization and talent data for group members

Gives equipped and overall iLvl to indicate the quality of a character's gear.

Inventory and bank organizer.

Bang!_Libram_Swap

GridLayoutPlus

Displays a variety of useful stats for tanking

Remaining and rested XP Data Broker plugin.