User interface prototyping is supposed to be a creative discipline, where the tools don’t get in the way, so you can place your ideas on the screen just like you would draw them freehand on the back of a napkin. Up until recently, there was not a high quality open source UI prototyper. But,those days are in the past, though, thanks to Pencil.

Attention computer animators -- if you've ever felt limited by working in three dimensions with tools like Blender, check out Pencil, an open source, cross-platform animation app that lets you create in glorious 2-D. Pencil mimics hand-drawn animation techniques, but it's easy to use and produces high-quality output.