I finally tried out BitTorrent, Bram's P2P file sharing app. The clever innovation in BitTorrent is that it separates the search for files from the download of files. Unlike KaZaA or Gnutella or whatever, BitTorrent only supports file download, not search. You just use the web to find the .torrent locator file you need for download, for instance for the Half Life 2 trailer.
BitTorrent is pregnant hackerware - it works great but is still poorly documented. And the apps aren't well integrated. Someone could build a killer tech company out of it. Some extra info is available in Brian's BitTorrent FAQ and the BitTorrent Wiki.
Me, I started downloading by doing
apt-get install bittorrent