I had an earlier post describing free or cheap UI mock-up tools.
To take things to the next level and start creating prototypes, I was looking for rapid application development (RAD) tools that would allow me to draw out HTML pages with dragging and dropping.
I haven't yet found a suitable RAD HTML editor, but I did run across a blog post which provides copy-and-paste HTML elements (source) which you can use to quickly cobble a UI together. Here's that link :
Even better, that site provides a CSS-based style that will wrap your HTML designs in a rough, back-of-envelope style.