Top Ten

1. Argument Mapping Tutorials.  Six online tutorials in argument mapping, a core requirement for advanced critical thinking.
2. The Skeptic's Dictionary - over 400 definitions and essays.
3. The Fallacy Files by Gary Curtis. Best website on fallacies.
4. Butterflies and Wheels. Excellent reading - news, articles, and much more.   
5. Critical Thinking: What It Is and Why It Counts by Peter Facione. Good overview of the nature of critical thinking. (pdf file)
6. Of the Liberty of Thought and Discussion by John Stuart Mill. Classic chapter, densely packed with wisdom about thinking.
7. Chance - best resource for helping students think critically about issues involving probability and statistics
8. Psychology of Intelligence Analysis, by Richards Heuer. A good overview of how to improve thinking in the light of insights from cognitive psychology. 
9. A Handbook on Writing Argumentative and Interpretative Essays by Ian Johnston 
10. Baloney Detection Part 1 and Part 2 - by Michael Shermer. 10 step guide.