Armory

In short

All armory information as (once) seen on the official site from Blizzard Entertainment, in game, for all your characters, on all realms
Enables the use of a compact character frame
Detailed profession information of all your characters
PVP information, including arena teams
Searchable inventory, including bags, bank, mailbox and auctioned items
Quest logs and spell books of all your characters
Friends, ignores and events of all your characters
Share quests, recipes and character information with other Armory users
Searchable Guild Bank that updates itself automatically without visiting the bank yourself
Tooltip enhancement and equipment comparison tooltips
Customizable functionality
And much more…

(Some features may not be available in the wotlk version)