Bagnon is the most popular replacement of the bag interface, designed to help the player find items as quickly and easily as possible.
It organizes all bags into a single window, highlights important items, sorts on request and helps you find them using a search engine. It supports the inventory, bank, void storage and guild bank, which can be viewed at any time, from anywhere and from any character! Bagnon can fit into any UI and player, as both its appearance and behavior is highly customizable. Many more advanced features are also available, such as tooltip item counts, item cleaning, and DataBroker support.
If you're looking for more features, many different plugins have been made by the community. If you are a developer, Bagnon provides a rich API. If you want instead Bagnon to look more like the default interface, try Combuctor.