Is in the sources.
My second choice is Flush. Similar to the above, but lacks some features and has a few quirks as if still at its beginnings. But most of the time is very good and has the lowest memory use of all.
sudo add-apt-repository ppa:konishchevdmitry/flush sudo apt-get update sudo apt-get install flush
Tixati is a very interesting and reach program. Maybe the best for advanced action, with a bit more memory use. To get it, here.
Deluge, one of the most popular clients in Ubuntu, deserves to be put at hand. To install, use Synaptic, etc.
I would not recommend and I would look for alternatives to:
Ktorrent. Recommended by some users. I tested it and it has odd features (cannot close it from the tray icon; has tabs that serve no purpose; once created, they can be closed only by a separate clumsy button); the memory use is high; speed seems lower than in alternative clients I changed it for; it was this one that made me look for an alternative (and so I found the first three choices above).
Transmission. The default client in Xubuntu 11.10 is from my experience not the best choice (although people use it - in the same manner some Windows users stick to the default media player or web browser). It seems poor and the lack of features is not compensated by low memory footprint.
Bittornado. The same problem as above. Lacks a lot of features that are elementary if you ever used utorrent.
Vuze. Cannot recommended because of its huge memory use compared to all the rest. It is said to be full featured but, from my standpoint, its use of Java makes it bloated. On newest computers with more than 4GB of RAM all that would of course be irrelevant. Again, the benchmark set by utorrent makes me see things in this way.
By the way, utorrent in Wine is not too good.