The P2P concept and the original BitTorrent client have created a phenomenal change in how people share digital data.
But the original client is also infamous for its bulkiness and drag on system resources. Since its release, there has been numerous attempts by others to release compatible, better clients. Some (very) good examples include Azureus, BitComet, and a few others.
But the best of the best, compatible BitTorrent client is (as of today) µTorrent. Fast, efficient, small in size (at only 170KB!), lightweight, free (as in beer), excellent are some of the popular words people use to describe it. A nicely created piece of computing art.