Here's a unique take on an under-desk trolley, which offers storage, a little extra worksurface for your desk, a bag hook, and an extra seat, in a pinch.

The C9 Trolley is by Washington-based Watson Furniture, who writes:

"Its steel tray can easily convert from a storage hub into a convenient side table or a desk extension, ideal for spontaneous meetings and team collaborations. Beyond storage, it doubles as a practical seat for quick discussions or shared workstation tasks. The C9 Trolley brings mobile storage out from under the desk and into the open plan."

The tray can lock in the upper position, so you can use it as a tow handle.

The file access seems a bit inconvenient, but perhaps it's good enough for rarely-accessed reference stuff.

Made of steel, the C9 is available in a variety of powder-coated finishes.