Whimsical Wireframes - Lightning fast wireframing with real-time collaboration.
UX Flow Wireframe Prototyping System - Create beautiful user flows prototypes for your next project
Wireframe.cc - Sweet and simple online wireframing tool.
draw.io - Online diagramming application
Wireframe for Figma - Lightning fast wireframing on figma (500+ elements)
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.