Sketch Mirror - Preview your iOS designs directly on your devices
SketchBook - SketchBook is a professional drawing or painting application that allows designers, architects, and concept artist to sketch their ideas and create adorable illustrations in no time.
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.
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.
Flowkit - Sketch library for user flows/content maps/annotations
Artrage - A simple but lovely natural media painting and sketching program.