Back in my freelancing days, everyone was doing php3 web development. I did, too, and it worked out extremely well for me. A very popular market might be satur

Back in my freelancing days, everyone was doing php3 web development. I did, too, and it worked out extremely well for me. A very popular market might be saturated, which is a downside. But it might also mean that customers already know that they want/need exactly this, which makes finding projects extremely easy. If you want to have the best possible salary, go for an obscure niche where you can be the best in the world. If you just want an OK salary with not too much work, go for the most popular project categories.