Logisim

Logisim for Windows is a learning tool that helps students learn the basics of designing and simulating digital logic circuits. This software is free and open source (GPL). This software makes it easy to create your own circuits and watch them being simulated as you build them.

There are many different tools and views that make this a great way to learn. You can use the wiring tool to draw horizontal or vertical wires that automatically connect to components and to the other wires in your circuit. The combinational analysis module lets you convert between circuits, truth tables and Boolean expressions. You can use smaller circuits that you have already created as subcircuits inside of a bigger circuit rather than having to recreate the smaller circuit over and over again. Wires are color-coded so you can easily see their state while simulating and debugging, and the included components cover inputs and outputs, gates, multiplexers, arithmetic circuits, flip-flops and RAM memory. With these parts Logisim can be used to design and simulate entire CPUs for educational purposes.

You are able to save your work to a file, export it to a GIF image, or print it out to hand it in to your professor and show what you are able to create. Logisim is used by students at colleges and universities around the world, in classes ranging from introductory computer science to computer organization and computer architecture courses. It runs on any machine that supports Java 5 or later, and special versions are released for Windows and macOS.

Logisim is a simple learning tool that lets you build circuits and simulate them as you go to see how they work.

This is a widely used educational tool.  Note that its author suspended further development in 2014, so no new versions are expected.