London Drawing Classes
My very first site, and therefore one which really is an example of a site dominating a niche market: drawings and drawing classes. Because of the popularity of the site, it was expanded to include a drawing gallery, as well as a system which allows organisations to register and publish classes.
The Sites
The site is currently being expanded to include other major cities around the world.
It's a typical example of what makes a site work: user contributions facilitated by ease of use and a niche audience.