Uxcel - The easiest way to learn UX/UI design
Responsive Viewer - View multiple screen dimensions in one view
uxtoast: Learn UX Design - Become familiar with design laws and learn how to use them.
Responsinator - Designed to test responsive websites on different devices.
Awwwards Academy - Leading thinkers teach digital talent on awwwards academy
Responsive Website Test - Test any website on desktop, mobile, and tablet