Logisim is an educational tool for designing and simulating digital logic
circuits.

Features:

* It is free! (Logisim is open-source (GPL).)

* It runs on any machine supporting Java 1.4 or later; special versions are
  released for MacOS X and Windows. The cross-platform nature is important
  for students who have a variety of home/dorm computer systems.

* The drawing interface is based on an intuitive toolbar. 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!

* Completed circuits can be saved into a file, exported to a GIF file, or
  printed on a printer.

* Circuit layouts can be used as "subcircuits" of other circuits, allowing
  for hierarchical circuit design.

* Included circuit components include inputs and outputs, gates,
  multiplexers, arithmetic circuits, flip-flops, and RAM memory.

* The included "combinational analysis" module allows for conversion between
  circuits, truth tables, and Boolean expressions.