Wireframing (http://en.wikipedia.org/wiki/Website_wireframe) has been used as a quick prototyping solution to enable the exchange of ideas and discussions on look and feel features, between clients/contractors, visual designers/developers, etc.
We have used a number of tools through the years, from Microsoft Word to Microsoft Visio to Adobe Fireworks (graphic representations) or Photoshop, however the latest which has proven totally versatile is Balsamiq Mockups (http://balsamiq.com/products/mockups). It is a mix of MS Visio and a graphics tool, but the hand drawn effect for the mockups provides that feel that this is actually a mockup not the final tool.
Exporting to PDF saves a lot of time since its a one-step package for the client. What I really miss is the ability to have annotations and notes on the mockups (which are not hand drawn), to explain different aspects of the design.
PS: What is especially interesting is that it is an Adobe Air (http://www.adobe.com/products/air/) and not a C/C++ based application.