There's one included in any standard installation called Transmission.

I myself like Deluge.

In case you don't like transmission (just doesn't have the features I wanted is all, still works fine) and wanted to uninstall it (thepiratebay usually automatically launches your primary torrent client, only reason I uninstalled it) the terminal command is:

sudo apt-get purge transmission-gtk

I only say this because I had trouble myself finding the package transmission is under in Ubuntu at first.