Free and Open Source Software

I know TiddlyWiki quite well but have only poked at Logseq, so maybe it's more similar to this than I think, but TiddlyWiki is almost entirely implemented in itself. There's a very small core that's JavaScript but most of it is implemented as wiki objects (they call them "tiddlers," yes, really) and almost everything you interact with can be tweaked, overridden, or imitated. There's almost nothing that "the system" can do but you can't. It's idiosyncratic, kind of its own little universe to be learned and concepts to be understood, but if you do it's insanely flexible.

Dig deep enough, and you'll discover that it's not a weird little wiki — it's a tiny, self-contained object database and web frontend framework that they have used to make a weird little wiki, but you can use it for pretty much anything else you want, either on top of the wiki or tearing it down to build your own thing. I've used it to make a prediction tracker for a podcast I follow, I've made my own todo list app in it, and I made a Super Bowl prop bet game for friends to play that used to be spreadsheet-based. For me, it's the perfect "I just want to knock something together as a simple web app" tool.

And it has the fun party trick (this used to be the whole point of it but I'd argue it has moved beyond this now) that your entire wiki can be exported to a single HTML file that contains the entire fully functional app, even allowing people to make their own edits and save a new copy of the HTML file with new contents. If running a small web server isn't an issue, that's the easiest way to do it because saving is automatic and everything is centralized, otherwise you need to jump through some hoops to get your web browser to allow writing to the HTML file on disk or just save new copies every time.