I was actually trying to donate some money to the people behind Transmission, a fabulous BitTorrent client for OS X, but it seems they are not accepting donations.
Therefore, my respect and recommendations will have to suffice, at least until they start accepting donations. Transmission is very elegant, designed exactly like an OS X application should be – it is one of those applications that improves the aesthetics of your system. The interface is very, very clean. This is the first torrenting client I have tried where I have found every option I need without searching for it – where should the download go, which port is open and what are the download caps. And more.
On top of this, it uses a very small amount of system resources.
Bottom line – if you have a Mac, if you download stuff using the bittorrent protocol, you should try Transmission. Now!