It is open source and can be easily extended with sprite packs (I recommend this one https://github.com/nathanielw/Android-Lollipop-Pencil-Stencils) and export templates (I recommend this one made by me: https://github.com/DaniGuardiola/pencil-material-template Demo: http://daniguardiola.github.io/pencil-material-template). It is based in svg and can be easily customized sprite by sprite. You can also create your own collection of sprites by modifying or joining existing ones. The software is being updated frecuently in Github by the community and it improves on every update. Some stability issues have been corrected since I started using it and that means the maintainers listeen to the users and solve bugs when needed. This tool can be used to prototype on different screen sizes and platforms, and if you have some decent coding and designing skills you can make your own export template to be useful for your needs or professional so that you can make a good impression when trying to sell a concept. Pencil is definitely the right tool for fast design iteration and I will continue using it for a long long time. I already recommended it to a lot of my designer and coder friends.

Pencil is a really easy to use program with a beautiful and useful GUI. From the very beginning you can start designing your project thanks to it's intuitive interface. It has some standard collections of stencils and templates that are good enough for simple designs but at the same time it allows you to add your own or third party ones. Also supports the creation of diagrams, which are very helpful show links between elements. And finally it has a lot of export options, with beautiful features like links on the design that when you export it to html, they actually work and send you to the different parts.

It's really easy to use, the sketch style widgets look amazing and it's open source.