Logisim-evolution is a completely free and open-source educational, cross-platform software for designing and simulating digital logic circuits.

Project highlights:
<ul><li>easy to use circuit designer</li><li>logic circuit simulations</li><li>chronogram (to see the evolution of signals in your circuit)</li><li>electronic board integration - schematics can now be simulated on real hardware</li><li>VHDL components (components whose behavior is specified in VHDL!)</li><li>TCL/TK console (interfaces between the circuit and the user)</li><li>huge library of components (LEDs, TTLs, switches, SoCs)</li><li>Supports multiple languages</li><li>and more!</li></ul>