KTorrent

KTorrent running on a KDE4 session. Courtesy of Wikipedia

A highly-configurable front-end for torrent downloads

Features

• Upload and Download speed management, including scheduling
• Supports UDP Trackers
• Can import partially downloaded files
• Can use IPV6

and many more. A longer list of features is available from Wikipedia.

A full description can be found on the project pages and the many screenshots demonstrate its flexibility. There is also a user forum.