Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
Sketch Mirror - Preview your iOS designs directly on your devices
Inkpad - Inkpad - Vector illustration app for the iPad.
Flowkit - Sketch library for user flows/content maps/annotations
Sketch - Professional digital design for Mac.
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.