Product Marketing Gallery - Examples of great in-product marketing to improve adoption
UX Flow Wireframe Prototyping System - Create beautiful user flows prototypes for your next project
5X - Build your own data platform without building anything
draw.io - Online diagramming application
Plectica - Easily diagram anything, together
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.