Windows/Mac/Linux (Firefox): FireTorrent's alpha-level release aims at letting anyone using Firefox start downloading torrents from any source, no separate software required. For quick or one-shot downloads, it's pretty darned convenient.
The add-on, technically in the alpha stage and pseudo-invite-only (the link below gets past that requirement, however), doesn't have any preferences to configure, and limits your upload speed to 15 kb/s, most likely to preserve Firefox's basic browsing ability while you download. What it does do is simply convert any .torrent link you click on into a download, handled by Firefox just as if you were downloading a file directly. So if you want to grab something and imagine you'll be done with the download by the time you're out of your browser, FireTorrent fits the bill just fine. Configurable preferences are expected to arrive in the next release, according to the developer.