Best Torrent client around. It doesn't display ads like bitcomet.
Just watch out, some sites are suggesting that you pay for utorrent, when it is free.
official website for uTorrent, The Best Bittorrent Client
but be sure that you do not install ask toolbar that comes with it!, and be careful on what you download!
Excellent program. You can choose which files to download so that you don't waste bandwidth on files you don't want. For example, I used it to choose emulated games that I wanted for the Game Boy Advance, which came in a pack of 96, since I only needed one. Makes seeding easier too!