Pencil is a free and open-source GUI prototyping tool that is quick, easy and works across multiple platforms. This is a good software ONLY for beginners. The problem in this is that you can't draw smoothly... even if you have a graphics tablet, it will be hard to draw in pencil. Pencil also supports any exports formats including PNG and HTML. When exporting to HTML you can even define links and behavior, allowing you to 'walk' clients through the UI mockup with buttons which actually work. Don't be fooled by the fact that it is free! With regard to Pencil, free does NOT mean inferior. Pencil also supports exporting documents into popular formats including OpenOffice/LibreOffice text documents. Review collected by and hosted on G2.com.
Pencil used to be a good software, nothing to dislike about it, and something I faced during the time, I had some problems with XulRunner which was being used from Firefox in Ubuntu, but that was easily fixed. Review collected by and hosted on G2.com.