Myostatin is a protein that, along with its associated gene, limits the growth of muscle tissue in some mammals. The Belgian Blue cattle breed has a natural mutation of the gene associated with myostatin that supresses the protein, resulting in lean and heavily muscled cattle.
A myostatin inhibiting drug called Stamulumab is currently undergoing testing for treating those with muscular dystrophy. If approved, use and abuse by human athletes will surely follow. (via siege)
Update: Stamulumab is no longer undergoing testing. But a pharmaceutical company called Acceleron is developing a similar drug called ACE-031. (thx, stephen)