Concept maps or mind maps as they are often called are a really useful tool for outlining pros and cons or different perspectives in a debate. The example shown here is constructed using Freemind.
FreeMind is a free Java based mind mapping tool available here:
If you have the Java Runtime environment installed on your computer, you can try out this actual FreeMind mind map by clicking here
Compendium - Personal knowledge base - free
Developed by the Open University, "Compendium is about sharing ideas, creating artifacts, making things together, and breaking down the boundaries between dialogue, artifact, knowledge, and data."
Download from http://compendium.open.ac.uk/institute/
Other mind mapping tools
You can find a comprehensive list of free and commercial mind map tools here