Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
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.
Zeplin - Collaboration app for UI designers & frontend developers
Flowkit - Sketch library for user flows/content maps/annotations