WowAce

Gives detailed descriptions of particular localities, as zones or instances, and provides optional TomTom integration.

LDB data source for herb, ore and skinning node data

Library for tracking cooldowns of other players

Shows a circle on the minimap to indicate spell range

GemCensus inspects people and records which gems they have socketed. Then displays which gems are the most popular.

MainAssist

KillingSpree

Lexan

Displays information about the current zone on the World Map, for WoW MoP Classic

FuBar_OutfitterFu

Doublerightclick casting, autolure and sound enhancing

Engineering data for Ackis Recipe List.

RaidInfo

Historian

FuBar_MonkeyQuestFu

A customizable view of friends and guildmates.

Shows activity for hidden ChatFrames by making their tabs flash.

Good old LibTourist-3.0 for WoW Classic Mists of Pandaria

Highly-configurable shield / absorb tracker.

Swiss Army knife for instances, including player tracking, ignore buttons, loot tracking, notes, and more.