Bat Houses for fun and profit
In 1999, I built myself a bat house. I took pictures and posted a step by step description of the process on the internet.
I made quite a bit of money off of that website. (a lot for a college kid anyway) by selling the instruction book through Amazon.com
The website is still up even though I cancelled my service with that ISP years ago. I still get a few sales from the amazon links, and I still get questions from strangers about how to build bat houses. I was even interviewed by the Discovery channel once about my bat house.
My bat house has been in disrepair for quite some time. The wind and rain pulled it from the ground and bent the concrete reinforced pipes. We have been considering re-constructing it this summer.
I am not an expert on Bats or bat houses by any means, but what I do know I am happy to share. It is amazing how many people are hungry for knowlege.
I suspect most nitche interest areas have a simularily hungry audience.