I've been a big fan of David Burn's Feeling Good. If you ever find yourself in a blue mood, this book is a goldmine of ideas.

In the book, he has a "Depression Checklist". You go through 20 or so questions, and get a score. This score can indiciate to you the severity of your mood.

I made this app to make it easy to take that test, and to save the scores. This way, you can look over and see how your mood is changing over time.