I've mentioned Honest Argument before, one of the group of websites that has sprung up recently to host online argumentation and which includes visual representation of those arguments - here is a better idea of what can be acheived:
Giveaway of the day are giving away ConceptDraw MindMap Personal (retail $119) for the next 18 hours. I haven't used the software, but the reviews are positive.
ConceptDraw MINDMAP 4 is a mind-mapping and team brainstorming tool with extended drawing capabilities.
Use it to efficiently organize your ideas and tasks with the help of Mind Mapping technique. ConceptDraw MINDMAP 4 supports extra file formats, multi-page documents. It offers a rich collection of pre-drawn shapes. ConceptDraw MINDMAP 4 has extended capabilities for creating web sites and PowerPoint presentations.