Utorrent's the best
utorrent. I am pretty sure that's what everyone is using.
utorrent/bittorrent official(which is now the same thing) if you're on windows, Azureus or ktorrent for linux, Transmission for OS X
Transmission for OSX. That ****s FAST, plus it auto-hides and keeps the upload/download speeds in the dock.
im a fan of piratebay