Features:
* Visualization of signal states with measurement graphs.
* Single gate mode to analyze oscillations.
* Analysis and synthesis of combinatorial and sequential circuits.
* Simple testing of circuits: You can create test cases and execute them to
  verify your design.
* Includes a simple editor for finite state machines (FSM). A FSM can then be
  converted to a state transition table and a circuit implementing the FSM.
* Contains a library with the most commonly used 74xx series integrated circuits
* Supports generic circuits. This allows the creation of circuits that can be
  parameterized when used. In this way, it is possible, for e.g., to create a
  barrel shifter with a selectable bit width.
* Supports large circuits: The "Conway's Game of Life" example consists of about
  2400 active components and works just fine.
* It is possible to use custom components which are implemented in Java and
  packed in a jar file. See this example for details.
* Simple remote TCP interface which e.g. allows an assembler IDE to control the
  simulator.
* Components can be described using VHDL or Verilog. The open source VHDL
  simulator ghdl needs to be installed to simulate a VHDL defined component, and
  the open source Verilog simulator Icarus Verilog is required to simulate a
  Verilog defined component.

WWW: https://github.com/hneemann/Digital