Critical Thinking on the Web, maintained by Tim Van Gelder of Austhink, a training and consulting group in Melborne, Australia, that specializing in training and consulting. Austhink's page on Argument Mapping looks interesting.

Critical Thinking Resources: An Annotated Bibliography, by Laura Bardroff Zieger from Montclair State. Although it's a bit old now (Fall 1998), you'll find quite a lot that's worthwhile.

Inquiry: Critical Thinking Across the Disciplines a scholarly journal published through the Institute for Critical Thinking at Montclair State University. The link will lead you to further descriptions, sample articles from past issues, and information on how to submit articles, and how to subscribe.
 
The Maryland Community College Consortium for Teaching Reasoning contains a number of valuable resources for teachers across campus in a variety of disciplines, such as articles, workshop handouts, lists of books on teaching students to think critically (including publishers' URLs and telephone numbers), and links to other web sites on teaching thinking.
 
Critical Thinking Community at Sonoma State This site will lead you to information about critical thinking at the college education level, and at the primary and secondary level. Sponsored by the Center for Critical Thinking at Sonoma State, this site is deep with useful documents and links that will help you begin to understand what critical thinking is all about and how teachers might use it in their classrooms.
 
The Ohio Center for Critical Thinking Instruction
 
ERIC/REC
 
UMass-Boston's Graduate Program in Critical and Creative Thinking