Freeplane - Freeplane is a powerful and free software for building mind maps.
Wondershare PDFelement - All-in-one PDF editor
Linked Ideas - A macOS/iOS apps to treat ideas as links of concepts.
Foxit PhantomPDF - Edit PDF files with our feature-rich PDF Editor. Download Foxit PDF Editor to convert, sign, scan / OCR & more. A speedy PDF Editor alternative to Adobe Acrobat.
vym - VYM (View Your Mind) is a tool to generate and manipulate maps which show your thoughts.
Google Docs - Create a new document and edit with others at the same time -- from your computer, phone or tablet. Get stuff done with or without an internet connection. Use Docs to edit Word files. Free from Google.