Transmission is a cross-platform BitTorrent client that is:

Easy.

Transmission is designed to balance power and usability. We've set the defaults to Just Work and it only takes a few clicks to configure advanced features like watch directories, bad peer blocklists, and the web interface. When Ubuntu chose Transmission as its default BitTorrent client, one of the most-cited reasons was its easy learning curve.

Lean.

With few dependencies and written in C, Transmission has the lowest memory footprint of any BitTorrent client we've seen. Imageshack chose Transmission for its BitTorrent farms because the competition requires amounts of memory several times greater than Transmission. The headless version of Transmission is the client of choice on low-memory hardware, leading many users to install it on their routers to leave running 24/7. Some commercial devices ship with it preinstalled.

Native.

Unlike many cross-platform applications, Transmission integrates seamlessly with your operating system. The Mac OS X interface is written in Objective-C and uses Growl notifications and dock badging to keep you informed.

It's fast, it's extremely lightweight, and — even though it's available for a variety of platforms — it behaves just as you'd expect a Mac program to.
Macworld, naming Transmission a Mac Gem

The GTK+ interface has been carefully written to follow the GNOME Human Interface Guidelines. If your desktop supports popup notifications, desktop sounds, and a system tray, the GTK+ interface will make use of them.

Powerful.

Transmission has the features you want from a BitTorrent client: encryption, a web interface, peer exchange, magnet links, DHT, UPnP and NAT-PMP port forwarding, webseed support, watch directories, tracker editing, global and per-torrent speed limits, and more.

Free and Community-Driven.

Transmission is a volunteer project, so unlike some BitTorrent clients, it won't be "monetized" by becoming a media center or bundling adware, third-party toolbars, or other junk. Also, its source code can be reviewed by anyone and is available for reuse under the GPLv2 and MIT licenses. The development team welcomes anyone who is interested in contributing code, documentation, translations, or other help.