don't use bitcommet, I used to use it and some trackers don't let you use it anymore (something about leeching without seeding?). uTorrent is easily the best out of the many I've tried, and on my mac I use Transmission, which isn't very special, but very simple.

mininova.org is my favorite for things like this, it's much more accessible than some sites I've come across :tin foil hat: