Transmission is the default torrent client with Ubuntu. However, I love ktorrent because of the various features it provides. As I use ubuntu and KDE based softwares require kdelibs to be installed, I usually go for alternatives and I found deluge to be a great alternative for ktorrent. It provides similar features as ktorrent.
It can be installed directly via Synaptic but for the latest version, just type in the following in the terminal window. The following instructions apply to Ubuntu 9.04 (Jaunty Jackalope).