UX Flow Wireframe Prototyping System - Create beautiful user flows prototypes for your next project
Now UI Kit - A beautiful Bootstrap 4 UI kit. Yours free.
draw.io - Online diagramming application
Module 02: Layouts - Professional toolkit for UI/UX design
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.
Startup Toolkit - Interactive how-to guides to launch your startup