For weeding out developers from fakes, hit up codility.com.
I took the demo test and found it to be a fair challenge. Any developer mid-level and up should have no issues completing the test. They can choose from a variety of languages to complete the task. Hell, their language choice might even be a great topic during an interview.
I have sat through far too many interviews where the candidate had no business even applying – but somehow made it through the phone screens. From now on, CODE MUST BE WRITTEN!