Vuze (formerly known as Azureus) is a nice BitTorrent client. However I missed one important feature that other Mac OS X BitTorrent clients have: showing the upload and download rate in the dock icon. Since I missed it so much I sat down and implemented it myself as a plugin. Every two seconds the icon is updated.
The plugin is availabe for free under the liberal BSD license, so you can do almost everything you want with the source... as long as you give me credit.