Iron filings are small shavings of a ferromagnetic material. Ferromagnetic (for the purposes of this page, at least) means that they will align themselves with a magnetic field. That being the case, iron filings are an excellent way to display the magnetic field of one or multiple bar magnets. Just don't let the filings actually come into physical contact with the magnet or they will never come off.
There is also a (very limited) overhead projector option for this demonstration.