Blizzard is using the bittorrent protocol in there "Blizzard Downloader". Everytime a patch is released for World of Warcraft the downloader kicks in, connects to a torrent tracker, and starts connecting to other peers currently downloading the patch.
When World of Warcraft originially came out Blizzard was a victim of their own success. Using the bittorrent protocol shifted a lot of the bandwidth load from blizzard to individual users.
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