The old school way to do I/O on desktop was via the parallel port, which I imagine would still work if you get a PCIe parallel port card. For a more modern way, the FT232H chip supports UART, SPI, I2C, JTAG and raw GPIO over USB, and there are break-out boards available.

I was going to say this as well the GeekPort was a parallel port. I always thought it was marketed that way is that Macs didn’t have a parallel port. Having a parallel port would make the spec sheet seem regressive. I believe serial ports were always faster than parallel ports. I bought a BeBox at the time.

The original Nike store in NYC purchased one to do their signage media playback. It didn’t go well.

This is still an on-going thing for the LinuxCNC project (which mostly follows Mach3 in using a Parallel port interface to drive a CNC controller).