WowAce

Quest History tracker and shows lists of quests in each category

Try on different gear and see their total stats and their differences. In short, an ingame CT-Profiles-like addon.

Track time played, rested XP and money for all characters

AutoMagicly clicks though Daily Quests, and Turns them In.

Switch macro depending on zones or BigWigs boss modules.

QuestLog

Adirelle's unit frames, based on haste's oUF.

StatLogicLib

Questing minion

Keeps track of quests.

Bayi's Extended Questlog

Queing of /who and SendWho() queries and a much better interface (see docs), with gurantee to be executed & callback.

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

Tooltip of awesomeness. Moo. It'll graze your pasture.

Library providing mappings between trade skill recipes, spells and created items

Additional information about player spells.

Gears skins for Masque.

A simple combat log that displays events similar to SCT

Tracks vulnerability debuffs on multiple mobs

Displays online guild mates.