Sources of problems for teaching/tutoring young mathematicians

If you haven't been using "The Art and Craft of Problem Solving" by Zeitz, you should. Like, right at this moment. :)

For neat problems... you might want to look into "Which Way Did The Bicycle Go?" by Konhauser, Wagon and Velleman.

Mathcounts is aimed at talented middle school students. If you are affiliated with a school, you can register as a coach with them, and they can provide you with many helpful resources. If not, they still have problem sets available for sale from their catalog, as well as coaching handbooks, which include solutions and teaching advice.

Try Yakov Perelman, "Mathematics can be Fun".