This story from Wired News tells of a project to allow users to create functioning web site prototypes using a graphics tablet or mouse without the need for programming. The project is called Denim and is part of the work of the Group for User Interface Research at UC Berkeley. There's a nice video demo of how it works available at the Denim site and an example of a site sketched in Denim, but more importantly, there's a download available if you want to try it out.

Also associated with this group is the Open Prototyping project, which is an open source environment for developing user interfaces.