Freemind is excellent. I started using it I don't know maybe a year ago to put together a bunch of information for my game world. It's pretty amazing really.

If you are into writing anything you might want to check out yWriter5 as well. It is also free.

It is possible there is something better out there but these are the two things I used when I attempted the Nanowrimo challenge last year. That just turned out to be one too many hobbies for me I think.