The Mood Gym is a flash course that includes some diagnostic quizzes, explains some of the common patterns that people fall into, and provides tools for combating them. It is pretty lengthy (and a tad on the cheesy side,) so I wouldn't necessarily recommend it unless you either have an interest in cognitive behavior therapy, or struggle with depression or anxiety.
Here are some highlights for those of you who don't feel like going through the whole course. (You need to create a new account here before you can view these pages.)
* David Burns' Warped Thoughts
* Hints for Unwarping
* "Playing the Reporter (worksheet)
* Increase positive self-interpretations (worksheet)
* Testing thoughts & interpretations (worksheet)
* Experiment to prove or disprove your interpretations