Ed Felten from the Freedom to Tinker blog has written a post with Princeton senior Sauhard Sahi called Census of Files Available via BitTorrent. The survey takes a random sample of files available on a trackerless BitTorrent system. The article is full of caveats–discussion happening in the comments–but does dig into the likely copyright status of the works they found.
"[A]ll files that were available were equally likely to appear in the sample — the sample was not weighted by number of downloads, and it probably contains files that were never downloaded at all. So we can't say anything about the characteristics of BitTorrent downloads, or even of files that are downloaded via BitTorrent, only about files that are available on BitTorrent."
The final breakdown?
46% movies and shows (non-pornographic)
14% games and software
1% books and guides
14% could not classify