The last time a young entrepreneur executed his promise to revolutionize music with computer software, the industry sued him for billions of dollars. A decade after Shawn Fanning unleashed Napster, 26-year-old Daniel Ek is generating tidal waves across Europe with Spotify, a company that offers upward of 10 million songs, streamed free over the Internet. The difference this time is that Spotify is a legal service, sanctioned by the music industry -- to a degree.