Popular on eBay are old test equipment like Spectrum Analyzers, Oscilloscopes, and Multimeters. HPDisk by Gustafsson Anders creates a virtual GPIB interface that stores data to a SD-Card.This is done by emulating a special HP disk drive, that some HP instruments know how to write to when connected. (As Anders points out, this is not emulating a built-in floppy drive.)
Long before USB became the standard I/O interface in the computing world, bench-top instruments used HPIB. This was the “HP Interface Bus” used by Hewlett-Packard in the early days of their test equipment. It enabled instruments to share data and be automated by “desktop computers”.
Eventually HPIB evolved into GPIB and the IEEE-488 standard was created. Until about 2000, most instruments supported a true GPIB/IEEE488 hardware connector. Slowly that evolved into USB-based virtual connectors (like a virtual serial port).