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.
Uxcel - The easiest way to learn UX/UI design
Befront - Learn UX/UI design skills in practice with bite-sized tasks
UI Quiz - Test your design skills in 3 minutes
User Inyerface - A painful illustration of bad UX
UI Coach - Become a better UI designer in 30 days