Figma - Team-based interface design, Figma lets you collaborate on designs in real time.
UX Flow Wireframe Prototyping System - Create beautiful user flows prototypes for your next project
Whimsical Wireframes - Lightning fast wireframing with real-time collaboration.
draw.io - Online diagramming application
Invision - Prototyping and collaboration for design teams
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.