Insurance, work at home, bad credit, and jobs. These are good niches but the reason they are good is that there is a lot of competition, which means any web site you create in the same niche will be competing with all the others. You'll end up with a high paying niche and very little traffic. You can make more money with a less competitive niche and more traffic.
The best is on which you can write yourself and can share something which is useful for web users....if you just want to make money by making a site on a topic you know nothing, then most probably you will not be able to even make money with it
anything relating to money, insurance, lawyers, health, loans, credit, etc. will definitely make you a good amount of money
If you can create unique content and know what you're doing, then the health niches are the most high paying niches, I believe.
There are tons of niches that pay well, the trick is to find the ones that are not already highly competitive. Or even better, dig down and build your sites based on the long tail keywords in the high paying niches, then you can rank in the SEs, and still get great payouts.
Ya, it's a pretty low niche, but its worth it if you can bring the traffic. Gadgets = technology niche.