Logisim

Graphical tool for designing and simulating logic circuits.

Logisim is an educational tool for designing and simulating digital

logic circuits. With its simple toolbar interface and simulation of

circuits as you build them, it is simple enough to facilitate learning

the most basic concepts related to logic circuits. With the capacity to

build larger circuits from smaller subcircuits, and to draw bundles of

wires with a single mouse drag, Logisim can be used (and is used) to

design and simulate entire CPUs for educational purposes.