Back in the 1980s it would've been the Sony Walkman, but now Apple have cemented their reputation as industry leaders with their instantly recognisable music players. Such is its influence that the "iPod" has become the universal term for "mp3 player".
With a variety of designs, specifications and modifications available, it's not hard to see why it has personally affected so many people. iTunes is also helping to keep the (legal) music business afloat.