NicheGeek recently posted some examples of Unconventional Online Business Ideas. If you review this list you will see a bunch of nifty ideas here. Lots of people are doing these types of sites and carving out a nice niche for themselves.
There are a lot of these types of opportunities in the market. If you are looking to start a business and want to work from home, running a site like one of these is a great way to go. There are tons of such ideas that you can come up with. If you read through the NicheGeek article, you will see a common theme running through them all.
All of the ideas grew out of the personal experience of the people involved. This ranges from people who are dealing with the solution to a health issue, to people who are leveraging a personal passion. Some of the businesses have grown to be quite sizable – Greekgear.com has revenues of $1.9M per year.
If you are trying to think about how to start an online business, start with your passion(s). Most likely you have a real chance of bringing some value add to the business that way, and you will be in a better position to market it effectively. In addition, you will probably enjoy it more, and that will make it easier for you to continue to work on it while you are still trying to build your initial revenue stream.