Just a quick note that the standalone version has a number of bugs (e.g. plugins might not install, templates might not install, etc.). So if you get an error, try the Firefox based one which works very well.
I don't know if it fixes any of the problems deyan mentions (or which version he used), but 1.3 is about 2 years newer than the version on their older site. They really should delete the old copy of their website's URLs, and redirect to their new site.
It's a free tool and makes rapid prototyping possible. Also, Firefox as a platform is free and widely available. The app itself is fairly good; not as powerful as, say, Balsamiq, but still better than many other options (e.g. Visio).
You can make cross-platform standalone applications with Firefox too: use XULRunner to package your HTML5/CSS3 application. As matter of fact, Firefox itself is built on top of XULRunner.
The advantage of XULRunner over AIR is that your users don't need to have AIR runtimes installed.
I admit to finding the name a bit off-putting. There is an amazing low-fidelity, highly iterative, and agile-friendly sketching tool and it is called a real pencil. On gasp paper.
Not that I don't use digital tools too, but I am often surprised how rarely people go analog when it can be invaluable.
True but often you need to present your design in a more professional environment and throwing together a sample UI in a matter of minutes can come quite handy.
This page has a version that says "Compatible with Firefox 4 to 11"
I did use this a few years ago and it was pretty good. Think of it as a really stripped version of Balsamiq Mockups (http://www.balsamiq.com/)