(photo by CmdrGravy of flickr)

Step by step is a YUI-based tool that builds guided walkthroughs of Web applications.  This looks like a great tool to build documentation right into a Web application.  A context-sensitive help system could be built with this tool.  The script is available under a Creative Commons license.

The positives are it is much more interactive than a video or screencast and the users stay right on the Web site.   

The negative is that, at least for now, a programmer has to be involved in creating the experience.  A screencast or documentation page can be created by a nonprogrammer.  However, we could write a front end if someone wanted to use it extensively.

This seems like a great tool for introducing new users to your site.  Check out the examples.