: Claims to be the world's largest collection of open-access research papers.
: This is a proprietary format used to bundle multiple files into one "download-friendly" package.
If you are interested in the technology behind how playlists are created and managed (which often leads to them being packaged for download), consider:
: This comprehensive survey defines the "playlist generation problem" and reviews techniques used by services like Pandora or Spotify to curate music.
