Trilium Notes, continued by the community as TriliumNext, is a free, open-source note-taking app for building large personal knowledge bases. Notes are arranged in a tree with unlimited nesting, and a single note can appear in several places. It has a WYSIWYG editor with tables, images and maths, along with code notes, version history, per-note encryption, Excalidraw diagrams, mind maps, scripting, a REST API and a web clipper. You use it as a desktop app for Windows, macOS and Linux, or install it on your own server, for example with Docker, and reach it through the browser.
The software is licensed under the GNU AGPL v3. The original developer, zadam, handed the repository over to the TriliumNext community project; the sources name no company, no location and no parent company. The project runs no hosted service: syncing goes through a server you manage yourself, so you decide where the data lives. For European organisations this means full control, but also that administration, backups and security are your own responsibility.