Obsidian is a powerful note-taking tool.
It has a focus on visualizing notes as a graph of connected notes, allowing users to link and tag their notes easily.
It’s powered by Electron, which usually is just used by webdevs who are too lazy to make a native app, but in the case of Obsidian it’s actually good. Electron allows the app to be highly customized using CSS, allows plugins to be written in Javascript or Typescript, and allows for a wide variety of media within notes. Any media format that a typical web browser can handle, Obsidian can too. Images, videos, audio, you can even use things like iframes to embed external websites into notes. Also, some Javascript libraries are used in Obsidian like MathJax for advanced math formatting and MermaidJS for creating flowcharts.
Notes in Obsidian are written in Markdown which allows note writing to be simple and flexible.