FizzBuzz is an incredible filter for devs. Any kind of async JavaScript that needs to do something with the data after it fetches it has also been a winner. Lately the biggest weeding tool has been asking candidates to fetch some JSON from a bucket, then visualize that data. I'd say that maybe half the people manage to fetch the data in 30 minutes, and of that, maybe 10% get the data fetched within 10. The other 20 minutes is trying to help them recognize that their data is initially being rendered with "undefined" and they have to update the viz.