John Tarbox found this open source software for designing and simulating logic circuits known as LogicCircuit. According to the developer’s website, “LogicCircuit is free, open source educational software for designing and simulating digital logic circuits. It has an intuitive graphical user interface allowing you to create unrestricted circuit hierarchy with multi-bit buses, debug circuits behavior with oscilloscope, and navigate running circuits hierarchy.”
This is a Windows project and requires Microsoft.NET Framework 4.0 or higher.
Visit the project’s official website for downloads and more information.
Via the contact form.