WowAce

Allows the user to customize the the tooltip with lua-scripts

Adds tier set number and difficulty indicators to tier item tooltips.

Ratings

Never forget which one in that 5-mob pull casts what spell again.

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

LibQTipClick-1.0

Creates a broker plugin for all your broker launchers

A set of datatexts for ElvUI that provide you with detailed info about your professions

Item link modification

A system config addon to WOW

Really very early stage. No, earlier than that.

Calculates the relative value of attributes towards physical dps.

Shows what stat was reforged in item tooltips.

Tools to easily compare gear among party members.

Shows the Guild Achievements Points and Members in the tooltip

Very very simple tooltip

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

shows keystones from your alts, guild members and friends

Adds information about the items tokens can purchase and vendor location to tooltip

PimpMyProfession The Profession Guide