WowAce

Displays Instance Maps

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

Collects Herbs, Mines, Gas Clouds and Fishing locations and adds them to the worldmap and minimap

Adds various information based on PeriodicTable to item tooltips.

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

Track all threat redirection ability transfers and cooldowns (Misdirection and Tricks of the Trade).

Inspects all members of raid and saves in database, as well as lists people's dkp values

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

Keeps track of your current location.

Displays online guild mates.

Questing minion

Cryolysis 2

Test whether the player can really dispell a buff or debuff, given its talents.

XRS

Adds a status to Grid for debuffs applied by Raid Bosses.

Keeps track of your top hits, heals, criticals, etc.

Show the transmog appearance of an item when you mouse over it, and shows collection-status in bags, shops, and elsewhere. Works on transmog, mounts, pets, and decor.

Antagonist

Incoming Heals

Displays online friends.