Azureus is one of the most popular, and the one I use.
I installed it by compiling WxWidgets and it from scratch, however with some linux distributions you can find pre-compiled versions.
This advice is provided on the basis that you will only use the program to obtain bit torrents for which the copyright holder has given you permission.
E.g. StarOffice is published by SunMicrosystems using bit torrents.
