What are some niche markets most people don't know about?

I stumbled upon this article on people who make a living selling owl pellets to schools, and I can't help but wonder if there's other out-of-the-way markets like the owl pellet market.

I also found out about the market for gourmet mushrooms. Apparently, people can start their own small mushroom farm for a relatively low cost, and sell fresh mushrooms to restaurants and at their local farmers market.

What are some other markets like these?