Freeplane

Mind mapping is one of the categories most thoroughly colonized by subscriptions: a hundred nodes on the free tier, PDF export behind a membership, advanced styling behind a membership, sync behind a membership — for what is fundamentally a tree structure editor.

Freeplane is the veteran open-source answer, descended from FreeMind and traceable back more than two decades. GPL-2.0, 4,293 stars, and no node limits, no export limits, no feature walls.

The basic interaction is intuitive: Enter adds a sibling, Tab adds a child, and almost everything is keyboard-driven so your hands never leave the keys when ideas are coming fast. That matters for brainstorming — when the tool cannot keep up with your thinking, the thought breaks.

Where it goes beyond typical mind mappers:

• Conditional styling: define rules such as "nodes containing 'TODO' turn red with an icon", so the whole map reacts to its content — a decent lightweight project board.
• Formulas: nodes can compute from values in other nodes, handling budget estimates or effort totals without switching to a spreadsheet.
• Scripting: Groovy scripts can process nodes in bulk or generate structure automatically.
• Encrypted nodes: password-protect an individual branch, so a map mixing sensitive material can still be stored safely.
• Attributes and filtering: tag nodes with attributes and filter the display, which keeps large maps manageable.

Exports cover PNG, SVG, PDF, HTML, Markdown, OPML outlines and even a simple browsable web version. The file format is open XML, so nothing is locked in.

To be clear about:

• The interface is old-school Java desktop styling — dense with features, not visually refined. Next to commercial products it loses on first impressions; its value is depth and freedom, not looks. A presentation-ready map will need some styling work.
• It is a Java application with a bundled runtime in the official installers. Startup takes a few seconds longer than a native app.
• No official mobile app or cloud sync. For cross-device use, put the file in Syncthing or your own storage — a map is a single .mm file, so syncing is trivial.

For the occasional quick sketch, Excalidraw or draw.io are lighter. For running mind maps as long-lived knowledge and project structures, Freeplane has the most depth of any free option.