An online debate tool facilitates the act of debating by helping to manage the structure of argumentation. This distinguishes it from general purpose communication tools such as wikis and forums. Some online debate tools provide graphical representations of arguments, but this is not a requirement.
This wiki page gives a list and characterization of debate tools. Debate tools were previously discussed on the blog.
Note: Perhaps discussion should go on the discussion page, or the comments thread of the original post....