That is the 1541 adapter for the Go-64
emulator. It plugs into the Amiga's parallel port, and allows the emulator talk to a 1541 drive.
I still have mine, and use it with the CLI utilities from The A64 Package
to transfer files between my Amiga and C64.
If you are planning on using it, proceed with caution
. I believe there were two versions of that hardware, one for the A1000, and one for the A500/A2000. Make sure that you have the right one for your Amiga i.e., make sure that you are plugging it into the parallel port, and not the serial port.