Obviously you have the big three: BHP (BHP), Rio Tinto, and Vale (VALE). Vale is probably my favorite of the big three as I go hunting for one to short.
As you break out the smaller names, they get more locationary specific. In South Africa, you have Kumba Iron Ore (KIROY.PK). In North America, you have Cliffs Natural Resrouces (CLF), Labrador Iron Mines (LBRMF.PK), New Millenium Capital (NWLNF.PK), and the Mesabi Trust (MSB).
One play that I'd consider is buying put options on Vale with a Strike price of $25 that expire January 2013 for $2.75. Right now the VIX premium is low and that premium would surely go higher as iron ore prices fall.