Despite somewhat limited programming skills, I've given some thought to a few ideas for web applications. While none of these ideas have moved beyond the conceptual phase, I've found Balsamiq Mockups to be an invaluable tool for refining them.
Mockups is an Adobe Air based application that allows you to quickly build navigable wireframe versions of websites or desktop applications using a drag and drop interface. Thanks to a good number of built in controls, and an active community site that allows users to share templates, almost anything can be created within the program. After dropping and positioning the various elements onto the canvas, the user can easily link them together and click through the site in order to get a better understanding of how it might work. It's even possible to export a HTML version of the page, using a secondary program called Napkee.
Applications such as Mockups are extremely powerful because they let you visually communicate your idea without having to actually build a functioning website or program. They're also very useful for evaluating different interaction models and page layouts.
Mockups works very well overall, but there are a few areas that could use improvement.
Some users may not like the visual style of the program. The mockups themselves have a very rough, hand drawn appearance, and the much derided Comic Sans typeface is used throughout the application. In my experience, this was not a major issue since Mockups is more about the function of the idea rather than its appearance, but there are those that would probably appreciate a more professional design language.
Editing linked designs is another area that could use improvement. This may be different for other versions of the program that use some sort of backend, but the desktop version requires that the user manually link every element, even when those elements are the same across multiple pages. You can work around this by cloning pages that you've already created, but a more elegant solution would be nice.
In my opinion, the only major drawback to Mockups is that it's built on Adobe Air. While this allows the program to run on a variety of platforms, it sacrifices speed as well as native look and feel. The good news is that new releases of Air dramatically improve performance, and the developer of Mockups is constantly working towards making the program behave more like a native app. It still has quite a way to go, but new releases occur frequently and the company seems to have a genuine interest in making sure its customers are happy.
If you're looking for a rapid prototyping tool for application design, I would strongly recommend Balsamiq Mockups.