To find a niche, first consider your skills. Make an honest assessment; deluding yourself won't net customers in the long run. Do you have experience in merchant services, back-end CMS, or member services such as forums and chat rooms? Once you have determined what your capabilities and strengths are, look for a category of business or organization that fits your skill set and develop a website loaded with features to meet their needs.
One good example of niche marketing to consider is HOA (Home Owners Association) websites. HOAs are member organizations and need services that cater to their members and a way to communicate news and information to the neighborhood and potential buyers. A good way to start is by looking at HOAs online and making a list of common features, then expanding on that list to include more than what you can find on the web. Most organizations depend on member interaction, so an HOA website with a social structure at its base would be an easy sell. You might develop a newsletter formatting tool that produces an html output for mailing, a mailing program to send out to all the members, a services directory, an emergency alert mailing program, etc. Figure out what they need and offer it to them.