Figma - Team-based interface design, Figma lets you collaborate on designs in real time.
Sketch Mirror - Preview your iOS designs directly on your devices
Invision - Prototyping and collaboration for design teams
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.
Sketch - Professional digital design for Mac.
Flowkit - Sketch library for user flows/content maps/annotations