A lot of success can come from creating simple apps for undeserved niches. I've made about $10,000 over last two years selling an app that is literally 5 images in a tab view. It took me one weekend to make. I know it's not too much, but it feels good in the pocket, the money flows steadily every day no matter what and requires absolutely no maintenance or additional work on my part, ever. There's a lot of low-hanging fruit in the long tail of the distribution.