Marketers don't usually like to reveal their niches because of fear of competition. I will tell you this, though. I used to like working in the "make money online" niche -- until the landscape changed. Back-in-the-day, new products used to come out on a weekly basis. If a product was crappy (lots of them were), all you had to do what tell people why they sucked and recommend something legitimate. People appreciated the honesty. Now, I barely see anything worth promoting.
Besides that niche, I have worked in the "gardening" niche, "learn Spanish" niche, dating niche (still in this one) and in another one I won't reveal because I just recently got the site up and running. It's a physical product on Amazon. I've never actually promoted anything like this, so I don't know exactly how it will go. With that said, I'm also thinking about the Fall and Winter months (will be hear before you know it). I've got some ideas of things I can promote (based on last year's searches).
With that said, I haven't promoted anything like that stuff, either. I'm going to be learning a lot of the next few months, which is great. I'll be able to start the new year out with a bang. Going back to your original question, though, I'd recommend you start with your hobbies. Most people have hobbies that are in profitable niches. This will also make it easier for you to promote stuff you know about and help you to connect with the audience.
Joey
