Possessions

Possessions keeps track of gold and items across all of your
characters on each server. Its primary utility comes from being able to
use a slash command to instantly show you where you already possess an
item on the server without cluttering your tooltips all the time.

Commands:

• /poss – Open the possessions window
• /poss text – Open Possessions and search for “text”
• /poss [Item Link] – Open Possessions and search for the item linked

Related:

• ShaguInventory – this addon shows the count of items you have on all chars (account-wide) as a tooltip info, complementing the Possessions addon.
• FamilyLedger – an app for viewing in-game item data held in World of Warcraft accounts, even when offline (uses Possessions addon data files).