WowAce

How many quests do you have in your log? ;)

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

Displays friend notes in unit tooltips, at logon, and in /who results.

Healer gear benchmark

Shows the spec of people around you in the tooltip.

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

Makes your item stats shine!

This will keep track of how much you have paid per item in your inventory. Only one of (Auctionator, Auctioneer, AuctionLite,...

Dynamic item DKP calculation

Should I send that recipe to my alt?

Shows mana usage as a percentage in a spells tooltip.

A system config addon to WOW

Shows loot information in item tooltips

Provides shareable notes and other information for your guild, party and raid community.

LibDataBroker plugin showing some random XP related data

Shows the source of an item as a tooltip

StarTip (Tooltips from Outerspace)

Keeps track of Gimps and warns you if you end up grouping with them.

Adds tooltip for debuffs (and other things? you found out: no).

Shift-clicking on a user name in chat shows a tooltip with their whois information.