UX Flow Wireframe Prototyping System - Create beautiful user flows prototypes for your next project
Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping
draw.io - Online diagramming application
Platforma - Wireframe library of layouts and UI elements
UMLGraph - UMLGraph is a professional automated drawing tool that allows the designers the declarative specification and drawing of UML class and sequence diagram.
iOS Wireframing UX Kit - An easy, speedy way to wireframe for iOS