Chrome: BitTorrent clients aren't hard to come by, but if you don't want to deal with extra software, OneClick is a Chrome extension that downloads torrents directly in your browser. Simply click on a torrent file and it will start downloading the contents directly in Chrome.
I had mixed luck with getting OneClick to work. The extension's main page suggests trying a Counting Crows official torrent, and that worked fine, but OneClick didn't recognise a few different Linux torrents. It doesn't offer the features of a dedicated BitTorrent client, but it might come in handy for the occasional download.