Marvel + Sketch Prototyping - Your Sketch workflow, reimagined
UX Flow Wireframe Prototyping System - Create beautiful user flows prototypes for your next project
draw.io - Online diagramming application
ProtoPie - ProtoPie is the easiest tool used to turn your UI/UX design ideas into highly interactive prototypes for mobile, desktop, web, all the way to IoT.
yEd - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.
Prototypr Makerspace - Design and prototype your ideas with the community