Krita - Krita is a professional FREE and open source painting program. It is made by artists that want to seaffordable art tools for everyone. Concept art. texture and matte painters, illustrations and comics.
Sketch Mirror - Preview your iOS designs directly on your devices
Artrage - A simple but lovely natural media painting and sketching program.
Flowkit - Sketch library for user flows/content maps/annotations
MyPaint - MyPaint is a fast, distraction-free, and easy painting tool for digital artists.
Crystal: Sketch Mirror for Android - Crystal: Sketch Mirror for Android is an application that is designed for viewing your sketch designs in real-time, navigate prototypes and download content related to the sketches for offline viewing.