Easy Positive Audio is a fast and free app to play (and record if you have a microphone) happy thoughts and improve your mental health. Studies have shown that long lasting happiness is augmented when we are reminded of specific happy thoughts and events from time to time. Easy Positive Audio provides that reminder.
You can download it on the Android Market here.
Update 4/2012 This is now open source and hosted on GitHub. Check it out at my GitHub page.