Acquisition Pro will set you back $24, though there is a free version which has some interesting limitations. The free version will cap your download speeds after an hour, and some results in your searches will be randomly omitted.
We here at TUAW think BitTorrent is a cool technology, but must remind you that it shouldn't be used for downloading copyrighted materials. Stick with open source distributions or things which you know are not copyrighted and all will be right with the world.
Update: Folks have pointed out, rightly, that while Acquisition does BitTorrorent files it is more of a peer to peer client.