Windows only: Popular BitTorrent client uTorrent added a web interface, so you can remotely access your transfers from a web browser. Since then, they've added support for the web interface to the iPhone and, more recently, iPad and Android devices.
Among other bug fixes and small feature additions, the new alpha version of uTorrent improves the web interface feature, allowing you to access it not only from a web browser on any computer, but also from the iPhone, iPad and Android phones. All you need to do is set up a username and password in uTorrent under Options > Preferences > Web and then head to http://web.utorrent.com on your chosen device. You can even add torrents by URL through the web interface, which is pretty nice. Hit the link to check out the new alpha.
uTorrent is a free download for Windows only. If you'd prefer not to install an alpha build, you can still get access to the web interface by installing the experimental Falcon client.