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 an optional cloud sync, and the editor experience is polished.

AGPL v3, built in Dart/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, all features included. AppFlowy Cloud offers sync, collaboration, and AI features. The cloud tier starts free (limited storage and AI credits) with paid plans expected around $8-12/user/mo for teams.

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

The catch: AppFlowy is not Notion yet. The database views are simpler, the API is less developed, and the plugin/integration 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.