TkGate

If you're looking for a reliable digital circuit simulator, then TkGate might just be the answer to your needs. It's an event-driven simulator with a versatile graphical editor that is based on Tcl/Tk. With TkGate, you can design circuits that include a wide range of primitive elements. On top of that, you have the flexibility to create your own custom-made hierarchical designs by defining your own user-defined modules.

TkGate isn't just a complex simulator – it also provides a large selection of demo circuits and documentation that can be accessed through the "Help" menu. These example circuits come in handy when starting out with the software or for educational purposes. One of the most interesting demo circuits is a simple CPU that is programmed to play the Animals game.

The best part about TkGate is that it comes as a free software package with the source code and is distributed under the GNU Public License. This means that users have the freedom to modify and distribute the software as required. Overall, if you're looking for a comprehensive digital circuit simulator that empowers users to create custom designs and has plenty of documentation resources, TkGate is definitely worth a try.