Clutch is a WebUI for the Transmission BitTorrent client. It allows you to manage your torrents from anywhere you can access the internet, and runs on OS X and various flavors of *nix.
Clutch provides most of the basic features of the desktop client, including torrent upload, torrent start/stop, file path selection, speed limiting etc.
The Web interface back-end is written in PHP, with an HTML and JavaScript (AJAX) front-end. There is a native cocoa Clutch.app package available for OS X that provided a one-click method of managing Transmission torrents over the web.