Why would somebody go to MIT for a bachelors degree in music? Alex Rigopulos did, and, along with musician and engineering student Eran Egozy, the pair launched Harmonix Music Systems. After a slow start, the Cambridge company in 2005 published Guitar Hero, which came with a plastic instrument and let anybody pretend to play lead guitar in a rock band. It became one of the decades biggest games, and the more advanced Rock Band game that followed helped Harmonix rack up $3 billion in sales. The music game market has faded, but millions of fake guitars litter homes worldwide.