Prototypr Makerspace - Design and prototype your ideas with the community
UX Flow Wireframe Prototyping System - Create beautiful user flows prototypes for your next project
Inspr - Mock-up app designs right from your iPhone
draw.io - Online diagramming application
Fuse Studio - The world's first visual, real-time app development tool
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.