PersonalBrain is one of the better applications out there for this kind of task, allowing you to map your ideas very fluidly as you think. You can use PersonalBrain to create diagrams, networks or simply sketch out your thoughts. You start out with a ‘Brain’ (i.e. a thought), and then build it up into something more concrete. What makes PersonalBrain a bit better than many other similar apps is that you can even drag and drop files and web pages into the mind map to make it more useful and richer.