From writing basic HTML to building complex logic into mobile applications, the ability to smartly craft lines of code continues to be one of the most in-demand — and often, well-paying — skill sets one can have. So what skills are the most sought after? That's an ever-fluctuating, somewhat difficult thing to track. Normally, we'd avoid turning to a single source for such data, but its very nature makes Indeed.com an ideal place to look. The job search site aggregates more than 16 million listings from a wide range of sources, so it's fairly comprehensive.