The Virtual Math Tutor site contains solved math problems for high school and college levels.
Math Magic contains a bunch of mathematical puzzles that you might enjoy. The archive goes back to 1998.
Project Euler contains puzzles for math and programming. The intended audience would "...include students for whom the basic curriculum is not feeding their hunger to learn, adults whose background was not primarily mathematics but had an interest in things mathematical, and professionals who want to keep their problem solving and mathematics on the edge."