I posted some information back in August about GUI mock-up tools. Since then, I have tried a variety of tools, including the hosted Protoshare and the multi-platform desktop application Balsamiq Mockups. In the end I selected Balsamiq Mockups as the preferred tool to use.
The following shows an example of the type of prototyping that Balsamiq Mockups can deliver. It also provides the ability to automate the navigation to more easily show the flow and intended behavior of the application.

The following video provides a good overview of the tool and its features:
The reasons for selecting Balsamiq Mockups:
- fast creation of mock-ups
- intuitive and easy to use
- inexpensive
- free user-contributed extensions
- easy to extend with new media assets
- active community
- regular application updates
To make it easy to share the mock-up files within the team, we also decided to use dropbox.com. Dropbox allows seamless sharing of files with a team, provides backups of the files, allows multiple computer access…
Updates:
This blog entry has some good pointers to various templates and stencils that are useful for doing iPhone mockups – http://iphoneized.com/2009/03/roundup-iphone-stencils-graphics-templates/, as well as additional insights into iPhone mockup tools (both computer and pen and paper!) – http://iphoneized.com/2009/11/21-prototyping-mockup-wireframing-tools-iphone-app-development/

