From Carl Burch:
Logisim is a 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 to design and simulate entire CPUs for educational purposes. Its special versions are released for MacOS X and Windows. The cross-platform nature is important for students who have a variety of home and 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.
What's new in this version:
Version 2.7 added the option of switching the "explorer pane" between viewing the "toolbox" (the project's circuits and libraries that has appeared in the explorer pane in previous versions) and a simulation hierarchy.