BitTorrent is a peer-to-peer file distribution tool.
However, if you're currently (2006-03-27) running Sarge/stable, Azureus doesn't exist in Sarge. Instead, you can use BitTorrent or BitTorrent-GUI.
Related may be BTCompleteDir and BTCompleteDirGUI. However, I'm not sure I'd recommend them:
/etc/cron.daily/man-db: mandb: warning: /usr/share/man/man1/btcompletedir.1.gz is a dangling symlink mandb: warning: /usr/share/man/man1/btcompletedirgui.1.gz is a dangling symlink