UX Flow Wireframe Prototyping System - Create beautiful user flows prototypes for your next project
React Proto - React application design and prototyping tool
draw.io - Online diagramming application
UI Bakery - An intuitive visual internal tool builder. Allows you to create admin panels, CRMs, customer support tools on top of your database. Integration with MySQL, PostgreSQL, MongoDB, and more. Add business logic, manage user permissions, share your app.
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.
Invision - Prototyping and collaboration for design teams