From Caretta Software:
GUI Design Viewer is a companion product to GUI Design Studio that lets you view user interface designs and run your prototypes. And because they share the same engine, your designs will look the same and your prototypes will behave the same in compatible versions of both products. In GUI Design Viewer we can open any GUI Design project (.GDP) or individual design file (.GUI) and opens specially created single file project distributions (.GDD files) such as the sample designs available on this site. It supports the same visual style and color scheme options for visualization. We can zoom and pan designs with the mouse and view design notes and annotations, including popup descriptions.