Azureus

From the site:

"Azureus is a BitTorrent client written in Java. The BitTorrent protocol is a new way of exchanging or distributing data over the internet. Downloading also means uploading, and the amounts of each are linked, to ensure fairness and rapidity in the spread of the file at hand. To be able to download a file, you first need to get the associated .torrent file. This file, usually a dozen KB in size, is the “signature” of the much bigger file to be downloaded, and it needs special software to be read properly. Azureus is one of those types of software. If you want to host files yourself, you need a tracker, which is basically a central server coordinating the connections between peers. Azureus can provide a tracker too."

Azureus

Dowload the Vuze client (for Mac or Linux)