Brain Teasers for Coders?

calvinandhobbes asks: "There are about 200 people working on different projects and most of them do programming without having an idea of what they actually do. they have little understanding of OS internals and primitive hacking skills. I want to enthuse them by providing some challenges by which they touch upon complex computing concepts, while solving the problem. Does anyone know of a set of C-based hacks or puzzles with which I can enthuse these budding programmers and testers?"