Logisim

Logisim is an educative tool that is free and open source, designed to help develop and stimulate digital logic circuits. This application, which was developed with an interface in a toolbar format, helps to facilitate learning the basic and most important concepts related to logical circuits.    

This tool graphically simulates circuits as you build them, so you do not have to complete the project to see if one of the first steps are wrong. It does not matter what the scope of your work is,    in Logisim you will be able to design circuits of great proportions to tiny sub circuits. You will also be able to draw bundles of cables very easily, just by moving your mouse across the worksheet.

The program, completely developed in Java, is a very valuable resource for students studying electronics. But the possibilities of Logisim do not fall short, which is why it is often used by academia for the design and simulation of whole processors and CPUs.

With all the tools that are available, it is worth highlighting the combinational analysis module to make conversions between circuits, Boolean expressions and truth tables. You will be able to save projects created with Logisim as .GIF files, or even send them directly to print.    

Logisim is so complete that it will seem that it fell from the sky if you are an electronics student. Download it completely free and you will not be disappointed.