Uxcel - The easiest way to learn UX/UI design
Responsive Viewer - View multiple screen dimensions in one view
UX Challenges - Done reading about UX? Start doing it.
Responsinator - Designed to test responsive websites on different devices.
uxtoast: Learn UX Design - Become familiar with design laws and learn how to use them.
Multi Screen Test - Test your site at different screen resolutions & devices.