The New York Times has an interesting writeup of a growing new trade built around services commonly used as smartphone apps: music-tracking, or music-identifying services. You've probably played with Pandora or Shazam on your iPhone already, and the article profiles just how all of that music gets tracked down and identified for your listening pleasure. A surprising amount of it is still done simply by human hands. While everything is still fed into the system, obviously (computers do have to know what the music is and what it sounds like), humans do a lot of the actual tracking down, both finding new tracks to index and implement, and listening in to describe to the computers how it sounds.
The other side of it is interesting, too -- Shazam's employees will keep an eye on what's showing up on television or in movies, and make sure that the database can identify any songs popping up and being tagged a lot. It's really wild. Of course I know that there are networks of people and computers behind these apps tI only use for a moment every day, but they're such a small part of my life that I tend to forget just how much work goes into them.