Last year I made a decent DJ application for Android. I probably invested a few thousand hours into it & the framework I built it on that could compile to Android, Blackberry Playbook, and HP Touchpad. I grossed just under $10k between sales on the three platforms, and advertising on Android. That works out to a few dollars per hour, last year.

This year, I don't have time to work on it further but between advertising and sales I'm still making ~400-600 USD/month. For 0 hours investment. I love the long tail.