7. MP3 + Napster
The audio engineers at the Fraunhofer Institute for Integrated Circuits weren't really trying to bring down an entire industry when they invented the MPEG-1 Layer 3 codec, better known as MP3. They were simply building on decades of research in audio compression, which started in the 1970s as an effort to deliver high-quality music over the phone.
But MP3 files' small size--roughly one-tenth the size of similar-quality .wav files distributed on CDs--was tailor-made for the broadband explosion of the late 1990s. The emergence of Winamp in 1997 made ripping CD tunes into MP3s easy, and the first portable players (the MPman F10 and the Diamond Rio PMP300) allowed music fans to listen to them without a computer. In 1999, Napster arrived, giving users an easy way to find new MP3s and share them--much to the chagrin of the Recording Industry Association of America.
Even if Napster didn't kill the record industry (my vote goes to the industry's own greed and stupidity), it changed the business irrevocably. Though the original incarnation of Napster was brought down by legal battles, it paved the way for peer-to-peer networking to develop as a legitimate distribution medium. If not for Napster, BitTorrent might not exist. Ironically, when Nine Inch Nails recently released its Ghosts I-IV album, it used BitTorrent to help distribute the first nine tracks free for the asking.
Disruption: The idea that media should be portable is disruptive. The notion that it should be free--and that some artists can survive, or even thrive, despite a lack of sales revenue--is even more so.