[IgorPartola]

Depends on what your goal is. If you want to make a lot of money, I'd say learn about an industry, especially one that's in a bad need of a technological overhaul. Make connections with people who can help you shape and advance a product. Almost nobody ever made money creating TODO apps, but plenty of people made loads of money bringing technology to where it was absent.

As for specific skills, sales and marketing are great ideas. If you want to be a very desirable coder, I'd say learning two technologies that don't normally go together but are both in demand at all times. For example, I have met very few people who have equal passion and skill for developing for iOS and Android. Add Windows Phone for completeness and most small companies would hire you on the spot because they can't afford a separate developer for each platform.

Another example of that might be front-end and backend work. Basically, if you can code up your own device drivers, then quickly create beautiful browser or mobile interfaces, you have become a truly full stack developer.

The ultimate triple threat would be: backend, front-end (web + mobile), and design. People that can quickly design beautiful things, then implement them are incredibly rare.

[Balgair]

While we are all here, lets make a list of some industries that are very badly in need of a tech overhaul. I'll start:

Any sport where girls are required to put bows in their hair. Ballet sure could use some carbon fiber thingy in those shoes. Same with figure skating. Gymnastics is a bit better, but I'm sure there is room for something.

Pipe and cigar smoking. The vaping trend hasn't hit them yet, but it will.

Road construction. It is terrible! Potholes everywhere!