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).
Big NOT fan of algorithmic question that can be summed up in one sentence taking a full page of text description.
In most of these coding test the algorithm is easy, but figuring out the catch in the description is harder than actual implementation (unless you're insanely limited and have to reimplement basics from scratch)
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).