I have been using Transmission but recently over the past few days it constantly crashes. I first thought it was the torrents fault but it does it whatever torrent I am downloading on.
Vuze has a Mac version. I use the Windows version so I cannot say for sure on the Mac version, but people seem to like it.
Best Free BitTorrent Client
http://www.techsupportalert.com/best-free-bittorrent-client.htm