Transmission is a relatively simple BitTorrent client. It's currently at version 0.61, but is generally stable and is certainly not devoid of features that matter. For example, it can automatically block peer clients that send bad torrent data, and it offers an Inspector palette for investigating individual torrent details.
The interface is quite similar to that of BitTorrent OSX, but it's more pleasant to use. It's clearly created by Mac enthusiasts, as it makes use of subtleties such as a dynamic Dock icon that shows current download and upload data rates, an Open Recent torrent list, icons for revealing downloading files in the Finder, and so on.
Many say Transmission is the best Mac BitTorrent client around, and we certainly found using it to be a pleasure. It's simple, but it's also intelligently designed. Unfortunately, reports indicate that it can be a noisy network client, and that's earned it disfavour in some circles.