uTorrent, or as it is also known as, microTorrent, is the Lightweight and Efficient BitTorrent Client (.2mb after install!!)
It is micro sized yet has all or most of all the BT features you will ever need. it is resource friendly. and when i say friendly, i mean it. This program barely uses any HD space and RAM usage is really low. Its skinnable and localized for other languages. Its also actively updated.
Best of all, its FREE!