The first Portable Personal Music Player. Up until this time, while transistor radios and portable cassette players existed, the concept of having the primary sound output channel be a headphone jack was revolutionary.
In 1979, Sony introduced the TPS-L2 Walkman portable audio cassette player. It was a stereo cassette player, but did not include radio tuning facilities and had a battery life reported to be 3 hours. The official rumour is that Stevie Wonder was one of the first people in North America to own one, as he had one shipped from Japan the day they were released.
Since then, the WalkMan line has expanded to include Radios, CD Players (through the DiscMan Line), DAT, MiniDisc and Mp3/Atrac/OpenMG players.