Transmission is one of the very best choices for a BitTorrent 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--and this open-source app provides a whole host of clever features and flexible settings, especially when it comes to managing your bandwidth.
Transmission 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, along with support for Growl notifications. As if all of that weren't appealing enough, Transmission is also free, making it a very attractive option for anyone who works with torrents. Recent updates have made Transmission faster (especially in the "endgame"), and added support for Quick Look and the ability to run a script upon download completion.
The current beta requires that you use an Intel Mac, but brings with it many cool improvements, including the ability to remove all completed transfers and to download the scarcest bits of a file first.