My Mega ST 4 "High" workstation uses an Eiffel interface, which lets me connect a standard PS/2 keyboard and mouse plus two joysticks to the machine's keyboard port. The original Atari keyboard and mouse have moved over to the Mega ST 4 "Low" machine.
The Eiffel is a free, GPL open-source project to connect a standard PC mouse and keyboard to an Atari ST, Mega ST, TT or Falcon. It was originally started by Laurent Favard with help from Didier Mequignon. Features include:
I use the PS/2 Atari Eiffel adapter with joystick ports from Klyde's Korner. This version connects to the keyboard port (RJ12) of an Atari Mega ST(E) or TT, and includes two joystick ports alongside the PS/2 keyboard and mouse. It's plug and play, comes pre-programmed with the latest firmware (v1.10.1Mq, which fixes a joystick firing problem), and needs no drivers. It connects via an RJ12 (6P6C) straight cable, and works with a native PS/2 keyboard and mouse or certain PS/2-compatible USB devices via a USB-to-PS/2 adapter.
Original Atari keyboards are getting rare and expensive. The Eiffel is a cheap, modern alternative that brings a standard PS/2 keyboard and mouse to the machine, along with the two joystick ports the original keyboard had. It makes the workstation far more practical to use day to day. The older Eiffel versions were originally built and sold by Lyndon Amsdon, with the design documented on the Atari Hardware Guide, and the project history is covered in the Atari Wiki.
In my Mega ST 4 "High" machine the Eiffel connects the PS/2 keyboard and mouse, keeping the workstation fully usable alongside the MonSTerBoard, ET4000 and NetUSBee. The original Atari keyboard and mouse are now used with the "Low" machine.