UX Flow Wireframe Prototyping System - Create beautiful user flows prototypes for your next project
draw.io - Online diagramming application
Prototyping Tools - Find the Prototyping Tool You Need
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.
UX Toolkit for Figma - Create beautiful user flow charts, sitemaps and wireframes
UMLGraph - UMLGraph is a professional automated drawing tool that allows the designers the declarative specification and drawing of UML class and sequence diagram.