"Last year, BitTorrent Inc. acquired the company that makes
uTorrent, one of the most popular BitTorrent clients available on
the Windows platform. The next major version of the official
BitTorrent client—which is currently in beta--is based on the
closed-source uTorrent client rather than the open-source
BitTorrent reference implementation.
"The decision to distribute a proprietary implementation as the
official client has generated controversy in the file-sharing
community. Some have expressed concern that new protocol features
included only in the proprietary official client will remain hidden
from the larger BitTorrent development community, making it
difficult for independent software developers to create
fully-functional competing BitTorrent clients..."