Uxcel - The easiest way to learn UX/UI design
One Click Test - Tool for quickly testing your design layout in one click.
uxtoast: Learn UX Design - Become familiar with design laws and learn how to use them.
W3C Mobile Checker - Make websites work better on mobile devices
Awwwards Academy - Leading thinkers teach digital talent on awwwards academy
Flexbox Grid - Grid based on CSS3 flexbox