Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.
UX Flow Wireframe Prototyping System - Create beautiful user flows prototypes for your next project
Figma Design Tokens - Making design tokens a single source of truth for Figma.
draw.io - Online diagramming application
Ditto for Developers - Sync the latest copy edits from design to development
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.