I was looking for a GUI prototyping tool that I could use to draft a new skin for a website and also for an Android application. I knew of Balsamiq but I prefer not to use non-free software when possible. I found Pencil Project, an awesome open source prototyping tool built on top of Mozilla’s XUL runner.
The Pencil Project’s unique mission is to build a free and opensource tool for making diagrams and GUI prototyping that everyone can use.
It seems to be working just fine for my needs. Awesome!