UX Flow Wireframe Prototyping System - Create beautiful user flows prototypes for your next project
draw.io - Online diagramming application
Freddy Feedback - Freddy Feedback is a tool to get feedback from your visitors and customers on anything that you communicate to them on your website.
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.
Prototyping Tools - Find the Prototyping Tool You Need
UMLGraph - UMLGraph is a professional automated drawing tool that allows the designers the declarative specification and drawing of UML class and sequence diagram.