WowAce

Auto repair + shows durability.

In-game theorycraft. Calculates actual damage/healing based on gear, talents and buffs.

One large inventory.

Bulk mail reception made really easy

A fancy LDB reputation display, show faction reputation and reputation change history

Provides access to all Portal and Teleport spells on right click of one button.

Achievements

Bulk mail sending made really easy

TomTom CrazyArrow pointing to the targeted group member

A library to determine estimated range.

Simple and super light-weight tracking of profession links for all your alts.

A customizable casting bar replacement for TBC Classic & Classic WoW

Library that tracks incoming heals

Database of item upgrade IDs

Lightweight Chat Improvements

Addon to manipulate the map

Minimap addon of awesomeness. *chewing sound*. It'll nibble your hay pellets.

Entertaining kill-shot notification.

Libra

Displays the remaing cooldown of the action you tried to use