Obsidian

Obsidian is a powerful, local-first knowledge base that uses plain Markdown files stored on your device. Its core strength is creating connections through backlinks, visualized in an interactive Graph View. A vast plugin ecosystem allows for extreme customization, from databases to Kanban boards. It's ideal for developers, researchers, and writers who value data ownership, offline access, and building a personal second brain.

The core app is free for personal use, with paid Sync and Publish services. It ranks highest for its unparalleled flexibility, privacy-centric model, and active community-driven development.