Logisim is great. But it's also discontinued, so I decided to search a bit for alternatives. And I found a couple that look very promising.
BOOLR is the more modern one, at least on the outside. Its site is flashy (but not with flash!) and its interface looks nice. It's made using Electron, which is a framework to write desktop applications using web technologies. It means it runs on Windows, macOS and Linux. And it's covered by the MIT licence (open source).
I played a little with it and it has some nice touches, like Ctrl+Mouse wheel for zoom and Ctrl-Z for undo. Or you can select part of your work and make a sub-circuit from it. I found the controls a bit strange though. Left-click will put the currently selected component. Or it will start connecting components with wires. Or it will change the value of the component. To connect two components you have to start from the source. And I constantly mix up the shift+drag (make a selection) with ctrl+drag (move the element underneath).
Overall it is fun to use it. I haven't tried any larger projects yet, so I don't know how it handles them.
Digital is inspired by Logisim. It looks similar, but not the same. It's written in Java (requires JRE 8) and again runs on Windows, macOS and Linux. It uses the GNU GPL v3 licence.
It looks and feels very similar to Logisim. And the author reports being able to clock a simple processor at 100KHz (compared to the 4.1KHz, that logisim supports), so it can be quite useful.
Both tools look promising and I hope their authors continue with their development.
P.S. There are a few project that try to continue Logisim's development. There are links on the Digital's page for the more notable ones.
P.P.S. The site alternativeTo is a nice source for alternatives for many software tools and libraries. I highly recommend it.
I finally managed to port my HACK implementation (except the keyboard) to Digital and it seems to work fine. I ran Mark's spiral and at full speed it's much faster that what you get on Logisim. It's comparable to, albeit a bit slower than the CPU Emulator!
I had to develop a custom display component, much like I did for Logisim. I will publish it on github soon.
Custom components support for Digital will be added in the next release, so I had to use the bleeding edge version.