Mindly is a free mindmapping tool that I have enjoyed and found quite useful for mapping out idea structures and relationships.  It has some solid output options.  Have to get the paid version to be able to open up more than 3 files and use the max number of elements therein, but this is a useful digital version of pen and paper brainstorming / idea-mapping.  It’s on Android, iPhone and Mac, with no PC version outside of running a program like Bluestacks (an Android emulator).   Maybe there are some PC versions that are better, but I preferred the clean and simple interface provided by Mindly.