I was going to recommend this one also. It's easy to use and keeps record of your moods over time. You can also use the Mania and Depression quizzes as needed to catch subtle signs that your mood is changing, for example I've felt "off" for a few days and restless/agitated, and as it turns out I'm mildly manic (which I characterize as hypomanic). It's good to keep in touch with yourself so you can do something about an incipient mood swing before it gets out of control.