Freeplane - Freeplane is a powerful and free software for building mind maps.
Okular - Okular is a universal document viewer based developed by KDE.
SuperNotecard - Introducing SuperNotecard. SuperNotecard is an online writing tool that features virtual notecards to help arrange facts or scenes, track details, organize paragraphs, and clarify your composition process.
Sumatra PDF - Sumatra PDF is a slim PDF/DjVu/EPUB/XPS/CHM/CBR/CBZ/MOBI viewer for Windows.
Linked Ideas - A macOS/iOS apps to treat ideas as links of concepts.
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.