[Broken link to Consultant Debunking Unit fixed - 5/29/02.]
Critical Thinking On The Web is an excellent site of quality links to websites about critical thinking, including the Skeptic's Dictionary and the Consultant Debunking Unit.
In the site authors' own words: "This site aims to gather in one place links to all the most useful critical thinking resources on the web. It certainly doesn't even try to link to every critical thinking-related page - just the ones that, in my opinion, are reasonable quality and worth a visit."
Link from Ex Libris.