SiYuan

SiYuan is a powerful, open-source personal knowledge management system with a block-based editor and an outliner. Data is stored locally in SQLite, ensuring privacy and performance. It features bidirectional linking, graph view, and supports Android. It's a strong choice for users seeking a free, privacy-focused, and feature-rich Notion-like experience with offline capability.

Development is community-supported. It ranks for its impressive feature set as a free, open-source tool and its commitment to local-first, portable data storage.