Here’s a small list of free BitTorrent clients for Mac OS X:

BitTorrent OS X - http://www.bittorrent.com/
Transmission - http://transmission.m0k.org/
Azureus - http://www.getazureus.com/
Tomato Torrent - http://sarwat.net/bittorrent/
Bits on Wheels - http://www.bitsonwheels.com/