1. A computer-implemented method of detecting relationships between digital works, the method comprising: receiving user-generated playlists from each of a plurality of player devices of each of a plurality of users, each playlist identifying a set of digital works selected by a user for playing; storing the user-generated playlists in computer storage; assessing a degree to which a first digital work and a second digital work are related to each other, at least in part, by determining a frequency of co-occurrence of the first and second digital works in the user-generated playlists, wherein said degree is assessed automatically by a system that comprises computer hardware; and storing a result of the assessment in computer storage.