I use problems with multiple acceptable solutions and easy tests of correctness -- i.e., it's obvious whether or not the approach is sound and the code works. For example, write a function that determines whether two rectangles on a display overlap, given the coordinates of their corners. Solving it requires the ability to analyze the problem and describe the conditions, the ability to code a simple Boolean predicate, and some aptitude for checking boundary cases and writing unit tests. I have others; what they have in common is that they're really easy problems for great programmers, and well within the limits of a 45-minute technical interview for competent coders, and good at exposing candidates who lack some basic skills.
