Just a quick note to remember. I've struggled so many times to get some good-enough web prototyping tool... just for specification purposes, mainly. I've played with plain-old HTML, powerpoint, visio, some specific-domain tools...

I intend to note down my findings, just trying to ease myself next time I'm forced to start the search. So here there are some quick links:

• Visio, the interaction designer's nail gun: My current favourite. It allows to design the features, without style-messing (leave that to a real designer). Background templates ease incremental development of your prototype. You can also export to a clickable HTML prototype, if you want.
  
• Denim: A domain-specific tool for "paper prototyping" dumped on to the web. Funny... includes screen-navigations and screen definition, and generates a clickable prototype. Checkout the demo-result here.
• A Visio wireframe stencil.
• Another visio stencil.