UX Flow Wireframe Prototyping System - Create beautiful user flows prototypes for your next project
Platforma - Wireframe library of layouts and UI elements
draw.io - Online diagramming application
Contra Wireframe Kit - Open source wireframe kit for design and prototyping 🚀
UMLGraph - UMLGraph is a professional automated drawing tool that allows the designers the declarative specification and drawing of UML class and sequence diagram.
Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping