WowAce

Allows customisation of login/logout system messages.

FuBar_MonkeyQuestFu

Prompts for canceling of pet battles if no rare pet is found in the opposing team.

Protractor

What did you just loot?

Enables the filtering of items when you visit a vendor

xchar

Tracking and ranking of dps/hps in a guild.

A full package of auras for all 3 Shaman specializations

Information display for Hunters.

mHud

Display the global cooldown around your cursor. (+ other stuff)

Lexan

Keeps a list of all items you want and shows how much you need.

Interface-central mount management

Timerbar embeddable library

Adds a couple of graph displays to Skada.

Fires events whenever a raid member "fails".

FuBar_uFriends

Keeps track of honor, PvP statistics, BG Marks, and Arena points.