Once upon a time, Mac users got the short end of the stick when it came to BitTorrent clients. Those days are gone, thankfully--and Transmission is a perfect example of that. This excellent open-source client is one of the very best choices for a BT client on the Mac, being both extremely lightweight (some users even manage to run the Ubuntu version on their cell phones) and fairly feature-packed.
Transmission's interface is easy to use--and easy on the eyes. The layout should be familiar, even to PC users, with torrent controls like stop and pause on the top, and torrent-specific information such as download speed next to the torrent's name. The app also provides a host of clever features, especially when it comes to managing your bandwidth.
The program lets you queue downloads, set "Speed Limits" for certain times of day, stop seeding when you've met a specified ratio, and more--all with live download stats viewable directly in the dock. There's support for Growl notifications, too, making Transmission a must-have for the Mac.