If you're building or rebuilding a Web site, there are several advanced and complex tools that can help you prototype your new product. There are also a few newer products that can capture the basics of design and let you share it with others--and that barely require more than a few functioning neurons to use.
At the most basic, there's the site-tree builder WriteMaps, which is so simple I initially thought it was a joke. It lets you create a tree of Web pages. It can also display your overall site in an outline view. I have never seen a simpler site visualization tool. But this one is so basic, most users will want to move beyond it pretty quickly.
Then there's Jumpchart, which makes it very easy to prototype the navigation and basic elements of a site, and to collect feedback from other people working on the design project. With this product, you can easily add pages and subpages to your site (as in WriteMaps), and you can also begin to plug content into the pages (which you cannot do in WriteMaps). Jumpchart uses a simplified markup language called Textile for content. That's good because it's really simple, like a wiki markup language, but bad because it's yet another coding scheme, and it's not WYSIWYG, which it really should be. Still, it does let you add images and fake forms, so you can visualize what your pages will look like, more or less.
Users can share Jumpchart designs with others and collect comments on prototype sites. On-page markup isn't part of the system, as it is on Octopz (review), ConceptShare (review), and Review Basics (review), but then those tools aren't site prototype builders.
You can't use Jumpchart to do advanced page layout or rich navigation design, but for sketching out the layout of a site, it's a really easy-to-use tool that I'm comfortable recommending. Jumpchart is still in closed beta but you can sign up for an invitation on the home page.