Debate 

• Critical Thinking on the Web
  Great set of links to sites about critical thinking, argument, assessment, hoaxes, evaluating sources, fallacies, statistics, and further related topics.
• IDEA Debate Database
  "Arguments for and against hundreds of debating Topics, written by expert debaters, judges and coaches. Also included are background summaries, links to websites of interest and recommended books, example motions and user comments."
• Debate Central
  From the World Debate Institute site at the University of Vermont, tons of information about formal debating skills and procedures.
• Guide to Parliamentary Debate
  From the American Parliamentary Debate Association.
• How to Debate Effectively and Rationally
  Although focused on debating political issues, the advice on this site is applicable to debates in general.
• A List of Fallacious Arguments
  Describes several common fallacious arguments. Here you can learn to avoid using these false arguments yourself and to recognize when your opponant is using them.
• McGill Debating Union: About Debating
  A brief introduction to debating from the McGill Debating Union.
• Online Debating Tutorial
  Short introduction to debating structure and technique.