Evil Mad Scientist Laboratories has quite a few neat things you can do with magnets. Some are really artful, some are great for teaching purposes, and here's one I'll definitely use sometime:
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Find studs in your walls
Move a magnet over the wall until it finds a screw or nail head under the paint. You don't even need to mark the wall-- you can just leave the magnet there until you've drilled your holes.
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Maybe not the best way to find a stud :-/