Peer-to-peer file distribution networks have been very popular in the last decade. Napster and later Kazaa have lowered the barrier to distributing (legally or not) music, while Bittorrent has made movie sharing very easy.
Music and movies aren't the only types of files that are distributed. As a side-effect, users can easily allow other users in the peer-to-peer network to access documents, spreadsheets and email archives, for instance. Often this is done without the knowledge of the end-user or with the end-user not considering the drawbacks of sharing his 'Shared Documents' folder. Confidential documents have been known to be leaked to the outside world via peer-to-peer networks, often with devastating effects.
The peer-to-peer monitoring project, or P2Pmon, was set up in order to monitor P2P networks for certain types of files. These files would then be automatically downloaded and stored, where the user of the system would be able to further inspect and classify the files.