I’ve all ways been fascinated by the use of networks to represent relationships among data objects. Recently I’ve been experimenting with using networks to represent ideas, concepts and knowledge models. FreeMind and Cmap Tools are two freely available and easy to use tools for doing this.
FreeMind only supports hierarchical layouts, but is very easy to use and make beautiful representations of complex ideas.
Cmap Tools is an incredibly expressive tool for making concept maps and knowledge models. I just started using it, but already the possibilities for content types and linked files is very impressive.