Beyond Compare - Beyond Compare allows you to compare files and folders.
vym - VYM (View Your Mind) is a tool to generate and manipulate maps which show your thoughts.
WinMerge - WinMerge is an open source differencing and merging tool for Windows.
Linked Ideas - A macOS/iOS apps to treat ideas as links of concepts.
Meld - What is Meld? Meld is a visual diff and merge tool targeted at developers.
Freeplane - Freeplane is a powerful and free software for building mind maps.