I just found out about a website. It may not be a challenge site, but perhaps will be a valuable link for self-learners.
It is user-driven and users add assessment tests and related questions. Each test is adaptive in that it becomes harder or easier as you answer. It is nice to see how you rank among others.
The UI is nice and it is free. One down-side is it's all multiple choice and there is not much interaction besides selecting an answer.