WowAce

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

Anchors the game tooltip to your mouse!

A system config addon to WOW

Shows the Guild Achievements Points and Members in the tooltip

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

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

Mob respawn timers

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

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

Item link modification

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

Show sorted reputation-list on FuBar-tooltip.

Calculates the relative value of attributes towards physical dps.

Show extra information about Jewels and Enchants in tooltips.

LibQTipClick-1.0

LCDs for WoW

Addon to add more information on a Tooltip to the GuildRoster

Sneakily inspect how much gold your mouseover target has made so far.

Shows what stat was reforged in item tooltips.

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