tiddlywiki

By far the most open, flexible and powerful knowledge management and development environment out there.

My second brain, full recommendation

TiddlyWiki works very well and I use it for spaced repetition memory.

Fantastic for personal website masters and Digital Garden Builders, very competent in managing fragmented pieces of short notes and quick reference information. Also good for practicing Personal Digital Zettelkasten (or Luhman Slip-box Note-taking method)

takes work to set up

good, but have learning curve

loved it for many years

TiddlyWiki is a fantastic, open source, hyper-extensible app which can also be a single html file. I have found it to be a joy to use and to adapt, and it has a friendly, helpful community of fellow enthusiasts behind it. For the past year or two, I've been using it principally to write and explore ideas on all kinds of topics, but it has also started to become the place I store contact details, track habits, plan my day, take reading notes, and learn through spaced repetition and retrieval practice. It is an incredibly versatile piece of software with fantastic hypermedia capabilities. THANK YOU to Jeremy Ruston for creating it, and to the super-helpful community which supports it, and which I learn from every week.

The learning curve is so steep you might want to drop it at the very beginning. But don't be deceived! Once you get a hang of how tiddlers work and set up a system that matches your workflow, the payoff is crazy