JDigitalSimulator is a plattform independend Electronic Design Automation software entirely build in Java. If you work for an educational institution or are a student, JDigitalSimulator is absolutely free of any charge.
JDigitalSimulator has a toolbar of build-in digital circuit elements, such as logic gates, flip-flops, switches, and displays. Drawing a circuit with JDigitalSimulator is like using a paint program. You click the element's icon on the toolbar and drag it to the position where you want the element to go. You use a similar procedure to draw wires and indicate connections. It's that simple.