Scapple

I find Scapple to be the easiest software to use to create low-friction, non-hierarchical diagrams and mind maps. It is simple, straightforward, and efficient. It's a great tool for mapping out ideas, processes, or writing projects. (Which is what it's made for, actually!) Scapple comes from the same folks who make Scrivener, the long-form writing package par excellence. They market it as for 'roughing it out,' and it really is better than anything I've found for doing just that.

I used it extensively while I was completing my doctoral dissertation. It was an easy, flexible way to do qualitative analysis code grouping (along with a dedicated qual study software package). I've also used it to organize to-do lists, personal kanban, take notes from professional presentations, etc.

Scapple is especially valuable as one of the few non-hierarchical mindmapping tools. Most of them require you to impose a tree-like hierarchy, but not so with Scapple. It has an infinite canvas (or at least can be expand to very large sizes) and can export to PNG, PDF, and other formats. You can create notes 'by hand' with mouse and keyboard, and it's also possible to import return-separated text documents and have it create individual notes for you. You can style the notes with different colors and fonts, make fonts bigger, bold, etc. You can also drag images into documents, or link to other documents, including to other Scapple docs. (I tried this for awhile, but have found it easier to just create single docs.)

FWIW, at least since @ship69's post, the Windows version has been updated. As of this writing, it's v. 1.4.2 (same as the Mac version). And they offer a generous 30-day free trial (30 days of actual use, not just calendar days). I've used it for at least 5 years and I have definitely gotten $21 of value out of it during that time. ☺

Scapple is an extremely simple/basic application which despite being a bit clunky has a very loyal following from a number of its users.

However new users might like to know that development of Scapple seems to have stopped and that the Windows has got stuck at version 1.0.0.0, and there have been no new releases since October 2013. The developers claim that they do not believe in minor releases and that at some point they will do some more development.