The first on-demand streaming service turned 20 years old last week. On Dec. 3, 2001, the San Francisco startup Listen.com launched Rhapsody--not a personalized online radio station but a searchable “celestial jukebox” allowing users to stream any song in its library for a monthly fee. At the time, this was novel and, depending on your connection speed, impractical. Two decades later it is the baseline of the music industry.