8 Open Source Second Brain Knowledge Base Tools
An app that helps you untangle all the complex thoughts running throughout the day can be considered a second brain. And, this involves allowing you to organize, structure, visualize, and recap all your ideas/thoughts that you took notes of.
Technically, not a lot of second brain apps exist. There's only a handful.
However, some feature-rich note-taking apps that give you the option to organize things using folders/tags/wiki, and you can utilize them as close as a knowledge management app. Here, I have listed some of them to help you out as well.