Stagsi is described as 'Hackable SQLite-based tagging, any file format, with scripts and C# plugins, powerful search query language, 100k+ files managed with ease'. There are more than 100 alternatives to Stagsi for a variety of platforms, including Windows, Mac, Android, Linux and Online / Web-based. The best alternative is Evernote, which is free. Other great apps like Stagsi are CherryTree (Free, Open Source), Zim (Free, Open Source), NoteLedge (Paid) and TagSpaces (Freemium, Open Source).
Polar is a powerful desktop app for managing all your web content, books, and notes. Polar keeps all your content in one place, supports tagging, annotation, highlighting and keeps track of your reading progress.
Nuclino is the easiest way to organize and share knowledge in teams. Create real-time collaborative documents and connect them instantly like a wiki. Use the tree, board, and graph view to explore and organize your knowledge visually.