Pencil is a free/open source prototyping tool. It gives you the ability to create prototypes of web applications and desktop applications and GUIs in the same way that Balsamiq does. It works on all platforms: Windows, Linux and Mac OS X.
I’ve personally used it to create web application product prototypes and also website designs. For me, the biggest advantage is how easy it is use and the built-in set of templates and stencils are great. It felt like I could prototype a whole app within a few days.
Creating a wireframe with Pencil is quicker and easier than trying to create a pixel-perfect prototype. What’s cool about Pencil is that it’s very easy to use and it can export each page as a PNG file which you can upload and share with others.
An interesting feature is the ability to export the whole set of wireframe pages as a set of web pages, making it easy for others to view. Combined with the inter-linking between pages that is possible, you can use Pencil to create a prototype that walks users through the different use cases and flows within your app.
Pencil provides stencils for Android and iOS so you can create wireframes for mobile and smart phone applications. Not only can you use it for wireframes and prototypes, you can also draw diagrams like flowcharts which are useful for documenting the various states and transitions within your app.
If you’re a developer you may be interested in the work the open source developers are doing on pencil, you can click here to check out their code repository.