The Godot Addon pack has just got a new addon, which provides a reasonably featured inventory system.
Some of the feature highlights are:
Bags automatically handle multiple slots and items. The amount of columns and rows can be specified from the Inspector.
Items can span across multiple rows and columns.
Special slots can be linked in order to easily manage "two handed weapons".
Special slots use internal filters to automatically allow or deny specified items.
A completely custom drag & drop system in order to allow for a finer control over when and how to start/end the process.
Items can have sockets.
Items on a bag can be automatically sorted.
Saving/loading inventory state into/from JSON data (binary support is in the TODO).
Inventory bag can be expanded or shrinked without affecting item placement.
Special slots can override maximum stack sizes to create specialized storing systems.
There is the OverlayDebugInfo script that provides means to quickly add text into the screen without the need to create UI controls all over the place. The resulting panel will expand/shrink according to its contents.
The addon pack tutorial has been updated to explain things about this new addon.