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 and tag gives a list and characterization of debate tools. Debate tools were previously discussed here.
There exists an academic literature on argument mapping and other tools (computer aided or not) for assisting debate. The most recent survey seems to be "Computer-supported argumentation: A review of the state of the art" written in June 2009, which lists 50 tools (starting on page 94)....