the humble but commercially formidable Sony Walkman
In June 1980 Sony Corporation of Japan introduced the "Walkman" into the American market and people's experience of music would never be the same. The cassette tape had been around for years but had found only niche audiences. Sony's co-founder came up with the basic idea for the Walkman in the late 70s when he was frustrated at having to carry around a bulky cassette tape player on business trips just to listen to music. He had a prototype developed and pitched the idea to the company's chairman as "...a good idea...".

It was that alright and a lot more.

The Walkman took off, eventually selling more than 200,000 units. If you didn't have a walkman in the mid 80s you just weren't happening. It would be like someone today who didn't have a mobile phone.

While the Walkman as it came to be known in the 80s and early 90s would eventually be shoved aside by the digital revolution it's indelible mark on popular culture cannot be over-emphasized. It made wearing  earphones in public acceptable, it cemented the idea that music was not something you had to listen to in a static indoor setting and it helped marked the beginning of the end for vinyl records.

I should note that while the Walkman product did lose it's pre-eminence in the market that doesn't mean it went away. Sony now makes high tech mp3 players sold under the Walkman name, as well as old-fashioned cassette player versions.