FYI Archived version of the original Bittorrent application (also known as Mainline, particularly to those developing other bt clients) are available here, back to 3.9.1.
http://download.bittorrent.com/dl/archive/
For example, I can confirm the BitTorrent-3.9.1-1.noarch.rpm contains the python source. The RPM format can be opened using common archive utilities. On OSX, I used Keka to unarchive to a cpio, then Keka again to unarchive into the file structure (I can see python scripts to do common bt tasks under usr/bin/ and the various code under usr/lib/python2.3/site-packages/BitTorrent )
When they switched to uTorrent codebase, the code became closed. As mentioned by others, there are many open source alternatives to the Bittorrent client offered by bittorrent.com.
Edit May 3, 2013: Bittorrent appear to no longer share their archive. You can still download the source via CVS, last updated in 2005. http://sourceforge.net/p/bittorrent/code/?source=navbar