At today's conference about Libraries, Teenagers and Web 2.0-yeah (slides at slideshare), I showed a quick example of how you can take an idea and rapidly produce it on the web. Sadly I was running out of time, so skipped over it a bit; I'm posting it here for a more thorough explanation.
Basically it was an attempt to show a non-techie audience how - with some existing building blocks (in this case PHP, Simplepie RSS and Amazon Web Services) - you can quickly build something engaging, once you have a solid concept of what problem you're solving.
The example started from the question: how to use the web to get people into libraries? The basic logic then follows these lines: people write blogs about interests, people tag blogs with topics and people want books about their interests. So build a tool to recommend books based on blogs and point people in the direction of somewhere to get them.
The recommend-atron takes a blog, goes though its syndication feed, pulls the top three tags and categories, then feeds them into a query to the Amazon's API, pulling back any relevant books. Amazon is used because they have the right hooks in place and I could get access. I'm sure you can get the same data from a library management system of any cop too.
Not particularly complex, and in the real world I'd use more than a simple query, but it shows how to go from idea to implementation in an evening and build something fun.