Logisim

Logisim is an open-source program that helps you make and simulate logic circuits. The tool is designed to be used in schools and can be used for creating both simple and complex educational circuits.

Interface
The tools user interface is easy to use, provided you know at least the basics of electronics and circuitry. The primary window would let you access all its tools and features.

Education
Logisim is used by students at colleges and universities around the world in many types of classes, ranging from a brief unit on logic in general-education computer science surveys, to computer organization courses, to full-semester courses on computer architecture.

Features
Color-coded wires aid in simulating and debugging a circuit. The wiring tool draws horizontal and vertical wires, automatically connecting to components and to other wires. It's very easy to draw circuits! The included "combinational analysis" module allows for conversion between circuits, truth tables, and Boolean expressions.