Obsidian is a knowledge base software for organizing notes written as markdown files. (Link)
Pricing
Obsidian is free. Paid services are offered for syncing your notes between devices, but you can also use
Alternatives
  • Notion and Coda. Notion and Coda are two paid note apps. Both have a similar set of cool features (hyperlinking, relational databases, automation), but they fundamental suffer from being proprietary software where you do not control your data.
  • Anki flashcard app. While Obsidian is great for storing complex notes and connecting together ideas, Anki serves as a useful way for memorizing useful facts us

After years of jumping between notes stored in different places (physical notebooks, Google Docs, Evernote, Notion, etc.), I finally switched to Obsidian. Each switch came with a huge headache as information had to be manually transferred to the new service. Notes in Obsidian are stored as regular markdown files that are stored locally and can be read and modified by any text editor. If Obsidian was to disappear overnight, I would retain my notes and a suitable replacement be readily available.

Within Obsidian, I organize my notes using the Zettlekasten method, with a relatively flat file structure.
I have a collection of “index” notes in one folder that serve as an entry point to my knowledge base and a collection of “Zettle” notes in another folder. Each Zettle contains (roughly) one idea or concept with links to other related Zettles. See “How to Take Smart Notes” by Sönke Ahrens for more information about the Zettlekasten system.