AppFlowy

AppFlowy is the closest open source alternative to Notion: docs, databases, kanban boards, and wikis, all self-hosted so your data stays on your machines. It runs as a desktop app (Windows, macOS, Linux) with optional cloud sync, and the editor experience is polished.

AGPL v3, built in Dart and Flutter with a Rust backend. The feature set covers rich-text docs, inline databases with views (table, board, calendar, grid), and a decent template system. Mobile apps exist but are newer and less mature than desktop. Self-hosting is free with all features included. AppFlowy Cloud handles sync, collaboration, and AI: the free tier covers two members and a small monthly AI allowance, Pro is 10 dollars per user a month, an AI MAX add-on is another 8 dollars per user, and a newer Vault plan runs 6 dollars per user for local, offline AI.

Solo: the desktop app with local storage is excellent and totally free. Small teams: you'll want AppFlowy Cloud or a self-hosted sync server for collaboration, and the self-hosted setup is Docker-based but not trivial. Medium to large: check whether the feature set is deep enough, since Notion's database formulas, relations, and API ecosystem are more mature.

The catch is that AppFlowy is not Notion yet. The database views are simpler, the API is less developed, and the plugin ecosystem is young. If your team relies on Notion's API integrations or complex relational databases, AppFlowy won't replace it today. But if your main concern is data ownership, it's the best option in this space.