Frontend Mentor - Supercharge your front-end skills by building in a real-life workflow. Solve real-world HTML, CSS and JavaScript challenges whilst working to professional quality designs.
uxtoast: Learn UX Design - Become familiar with design laws and learn how to use them.
Befront - Learn UX/UI design skills in practice with bite-sized tasks
Uxcel - The easiest way to learn UX/UI design
User Inyerface - A painful illustration of bad UX
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.