Freeplane - Freeplane is a powerful and free software for building mind maps.
Padlet - Visual boards for organizing anything.
Linked Ideas - A macOS/iOS apps to treat ideas as links of concepts.
BookWidgets - BookWidgets is an excellent application that let you create engaging exercises in minutes and easily share a varied variety of beautiful and fun activities with your classroom and others.
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.
Quizlet - Quizlet allows you to review and create flashcards for a variety of subjects, such as math and reading.