A great article from a couple years ago listing the top 50 gadgets ever made in the last 50 years - http://www.pcworld.com/article/id,123950-page,1/article.html

The original Sony Walkman TPS-L2 was listed as number one. Revolutionizing the personal stereo movement the Walkman paved the way for portable music players. The player had an orange "hot line" button when pressed would mute your music and activate a microphone so you could hear what someone was saying without removing your headphones. How cool is that.