UMLGraph - UMLGraph is a professional automated drawing tool that allows the designers the declarative specification and drawing of UML class and sequence diagram.
Whimsical Wireframes - Lightning fast wireframing with real-time collaboration.
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.
Adobe XD - Adobe XD is an all-in-one UX/UI solution for designing websites, mobile apps and more.
draw.io - Online diagramming application
ProtoPie - ProtoPie is the easiest tool used to turn your UI/UX design ideas into highly interactive prototypes for mobile, desktop, web, all the way to IoT.