What does HackerNews think of logisim-evolution?
Digital logic design tool and simulator
In addition to boolean logic, Logisim also includes more advanced components such as static RAM, as well as interesting things to drive with it, such as displays. It was developed for use in schools and is therefore designed to be easy to work (or rather play) with.
The project itself has been dead since 2011 or so, but there is an actively maintained fork [1].
Since then, the logisim project has discontinued, but it looks like there is a open source successor:
https://github.com/logisim-evolution/logisim-evolution
Have not tried it, but it looks promising, provided you don't want to do too complicated things (not sure if you could really model complex CPUs like a pentium with it). Also, it's pretty digital only, so I wouldn't expect Mac-Spice-like analog circuit simulation.