The Bit Torrent client µTorrent was written with efficiency in mind. Unlike many torrent clients, it does not hog valuable system resources - typically using less than 6MB of memory, allowing you to use the computer as if it weren't there at all. Additionally, the program itself is contained within a single executable less than 220 KB in size.
For newbies see here:
http : //www . utorrent.com/beginners-guide.php