These files are created by the BitTorrent P2P file sharing application, BitComet. When a download is started using BitComet (E.G mymovie.avi), a file is created with the extension .BC! (I.E mymovie.avi.bc!). Once the download is complete, the .BC! extension is removed. If a file still has the .BC! extension, it indicates that the download was not completed.