DFS Explorer, this allows the transfer of .ssd files to and from real disk over serial. In the 90's setup, I had a DOS boot disk in the laptop with a autoexec.bat (or was it config.sys?) set to redirect the console to the serial port, so I could use a terminal program on the BBC as if I was typing on the laptop (the BBC had a better keyboard), and would show the output from the laptop on the TV (since the latop screen was damaged and had been removed). Here is almost the reverse of that.
I initially had some problems, due to a cheap USB to serial adapter. The BASIC program transferred fine, but the handshaking is more stringent on the data transfer and it wouldn't go. The cheap USB to serial adapters are often very implemented and don't follow the RS232 spec very well. Once I switched to a proper serial port it was fine.
DFS Explorer, can be used as downloaded, or you can register for £5 to support the author, which I always like to do when I find something useful like that.
CBMXfer that I've used to transfer disk images from PC to a Commodore PET and a Commodore 64. Although in the Commodore case, the drives are intelligent, so the PC interfaces direct with the drive, rather than needing to use the computer as a slave. There is also something similar under way for the Spectrum, ZXTrans. The Spectrum is short of a serial port, so I am working on building a modern replacement for the Interface 1, to give a Spectrum a real serial port, or more likely, a USB connection with a microcontroller acting as USB-serial-Spectrum.