User Inyerface - A painful illustration of bad UX
UX Challenges - Done reading about UX? Start doing it.
Befront - Learn UX/UI design skills in practice with bite-sized tasks
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.
Learn Mobile Design - A curated list of mobile design resources
Weekly UX Exercise - Receive challenges top companies use to interview designers