Wow, HackerRank is amazing! Can't believe I've just heard of this -- I literally was going to build the same idea (with the same name) at a hackathon. I just wish they had more challenges in Javascript/Ruby.
Would like to see Cmd/Ctrl + Enter to compile/run tests when focused on editor. (or am I missing a similar keyboard combination.
Another problem was that I could not easily click a "Next" button to go the next challenge in a group. I would like to move onto the next functional programming question but I have to go back to the categories screen.
In general I think the list is really great.