For quite some time I have been searching for a nice tool to create mock-ups and wire-frames. There is Balsamiq Mockups which is of course a nice tool but at a certain degree you have to pay for it plus you have to install Adobe Air.
Eventually I found Mockingbird a online tool to create mock-ups and wire-frames. With this tool you can drag and drop, resize and rearrange user interface elements. Go from idea to mock-up in just a few minutes!
One of the Mockingbird key features is the ability to link multiply mock-ups together. When you link mock-ups you can simulate the real application and get a feel for the flow of your application.
Once you are done creating your mock-up you can easily share it with others by sending a link or convert the mock-up to PDF or PNG.
For more information check out the Mockingbird website.