I used to make mobile apps back before Android and the iPhone. These targeted Palm OS and Windows Mobile. I had some success and for a while was making anywhere between 400 and 600 USD per month, which as a supplemental income as a student was amazing. Ironically I think it was much easier to come up with a useful app that had no (or poor) competition back then with really basic mobile devices than what it is now. The market is flooded.