Sketch Mirror - Preview your iOS designs directly on your devices
AT-UI - UI library for developing powerful web interfaces
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.
Remake - A way to make web apps with just HTML & CSS
Flowkit - Sketch library for user flows/content maps/annotations
Sketch Extensions - An official collection of Sketch extensions & plugins