pDonkeyS is Perl implementation of an eDonkey server. The main goal of the server is to provide framework for further eDonkey protocol exploration, but it could be useful in testing of client software, since the server can be configured to output very detailed debugging info about eDonkey traffic.
Huntor is a full-featured command line Bittorrent client. It currently has no GUI. This client focuses on being run in the background, such as on servers. It is highly customizable. Its disk storage layout makes it easy to incrementally rsync the data to a different machine while downloading.