My sister bought me the book (thanks Tein) and I visited the site: Stuff on my Cat.
Cute kitties aside, the story behind the phenomenon is interesting. Mario Garza started the site in July 2005 because he liked putting stuff on his cat and decided to share his photos online. It took off! Millions of visitors, with many of them submitting photos of their cats, and write-ups in Yahoo! and GQ magazine.
What are you passionate about that you can share with the world? On a global scale, every interest is a massive niche.