WowAce

Alternate Rune Display

Keeps track of current money and all your characters on one realm.

AltClick an item in your bag to add it a mail, auction, or trade frame, and alt-click to buy a stack if reagents.

A simplistic clock.

Estimated time until current target will die.

Buffalo

Replacement for default bags.

Watch tooltip updates to announce when you find specific objects

NBuffBars

ShadowGreenLight helps you decide if recasting SWP/Corruption is worthwhile

Raid level alternative player power warning (Atramedes, Cho'gall, Lord Rhyolith)

Aspect checker addon for Hunters.

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

Keeps track of space left in your bags.

Pet Theory

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

Should I send that recipe to my alt?

A library to return spell duration and debuff type of a spellID.

Considers an item against all the items you have to determine whether or not it is an upgrade

BeneCast 10