Big fan of Codility. They do have a lot of algorithmic questions as well, but they also have easy ones (which we use), and we tested them on our own staff prior to deploying (all then-current staff got perfect or nearly perfect). I would only use medium or above if the question tested something that the job absolutely needed in a candidate.

They also allow candidates to add their own test cases easily, which, for the ones that do, tends to be a very strong signal in their favor (they also tend to do better).