Apps should not exists for the sole purpose of making their creators rich–they should genuinely attempt to solve problems and improve our lifes. tagtraum strives to to create such software. Apps that are beautifully designed, easy to use, and above all useful.
The prime example for this is beaTunes, an advanced music analysis and library management tool. It provides intelligent ways to fix music metadata and uses sophisticated algorithms to compute content-based data like tempo (BPM) or key.
Another example is BeatGauge, a very simple and beautifully designed tool that helps people determine the tempo of songs in their iTunes library.

Without free and open source libraries, most modern application development is unthinkable, because it’s too expensive. Leveraging existing work can increase productivity dramatically. If an available library is a good fit for the problem at hand: Use it.
But taking advantage of other people's work is only part of the story. Sharing your own libraries can be seen as a moral obligation and may even further your cause.
tagtraum offers the free Jipes library for audio feature extraction and several smaller libraries for Java-based audio decoding.

Unlike brick and mortar artifacts like bridges and roads, most software products have not been built a thousand times before. Each project requires both continuous, intensive communication and co-operation between all parties involved. Individual skill, experience, and agile methods can help to avoid the worst pitfalls. But the urge to understand problems and constraints, driven by curiosity, is still essential to arrive at satisfying solutions.
Additionally, a little scientific research may help.