WireframeSketcher 2.0: Introducing interactive prototypes
The latest version of WireframeSketcher adds support for interactive prototypes. You can now link two or more screens together and then make a click-through presentation using the builtin viewer. You can also export your prototypes to a clickable PDF document. This way you can send your prototypes to your clients without requiring them to have any software other than a PDF viewer.
See WireframeSketcher website for more details: http://wireframesketcher.com/