Hands down the best note taking app I've ever used. It's not open source but it's free. The notes are saved in Markdown (.md) format which can be read by any text editor; no proprietary file format. It works completely offline and the files are saved right inside your PC and easily accessible, no cloud only BS. You can sync between devices by subscribing to Obsidian Sync for $8/month, though personally I just use Syncthing instead. This way, all my notes never leave the local network. Obsidian has an Android app, but it's quite slow on startup, so I just use Markor instead. Obsidian also has a lot of extensions, for example Excalidraw. This extension helps a lot in designing rough UI sketch and planning.