Touration lets you easily create guided tours for a Web page
Touration is a simple to use web app for creating visual tours of your Web pages. If you go to their page and click Show me how it works you'll get a demo tour that would explain the concept in about 10 seconds, but the gist of it is that you get callouts that walk you through page elements, one by one.
They also offer a demo of the editor, which is really simple to use -- which is a large part of its appeal. Simply click a page element and create a rich-text callout for it; that's all there is to it. You can also add floating "dialog boxes" (i.e, explanations not directly related to any one page element), or "groups". Groups are basically callouts which highlight arbitrary sections of the page and not just one image or piece of text.
Note that this is a page tour, rather than a site tour. I think it can come in handy for sites with complex visual layouts or for web apps which pack loads of functionality onto a single page.