Pencil is an open-source GUI prototyping tool that's available for ALL platforms. Pencil is built for the purpose of providing a free and open-source GUI prototyping tool. Developed by Pascal Naidon and Patrick Corrieri
License: Open Source
Synfig Studio is a powerful, vector-based animation tool designed for complex animations, while Pencil is more suited for traditional hand-drawn styles and is easier for beginners. Synfig offers extensive features for detailed work, whereas Pencil focuses on simplicity and speed.