Outsourced IT. In most companies the IT department is an expensive bottleneck. Getting them to make you a simple web form could take months. Enter Wufoo. Now if the marketing department wants to put a form on the web, they can do it themselves in 5 minutes. You can take practically anything users still depend on IT departments for and base a startup on it, and you will have the enormous force of their present dissatisfaction pushing you forward.
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Online learning. US schools are often bad. A lot of parents realize it, and would be interested in ways for their kids to learn more. Till recently, schools, like newspapers, had geographical monopolies. But the web changes that. How can you teach kids now that you can reach them through the web? The possible answers are a lot more interesting than just putting books online.
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Off the shelf security. Services like ADT charge a fortune. Now that houses and their owners are both connected to networks practically all the time, a startup could stitch together alternatives out of cheap, existing hardware and services.