Mac only: When it's time to download something via BitTorrent, most Mac users manage their torrents with the popular client, Transmission. Folx offers almost all of Transmission's features, but adds folder organising, one-click torrent adding, and a download manager for your browser.
Folx has most of the features you've come to expect from something like Transmission: choosing the maximum number of transfers, proxy servers and NAT-PMP (for no-hassle port forwarding), but it also has a lot of less advanced (but very convenient) features built-in. The free version lets you organise your downloads by file type, meaning that movie files can go into a certain folder (say, your "Movies" folder) and audio files go into a different folder (say, "Music"). It also contains a number of ways to add torrents to the download list — instead of having to do everything manually, you can just double-click the link to any torrent file in your browser and Folx will automatically download the torrent and add it to your download queue. This is especially useful for browsers that don't support auto-opening of downloaded files, or websites that don't contain magnet links.
What's especially awesome about Folx is that it acts as a download manager for your browser too — just like adding torrents, all you need to do is double-click on any link in your browser and Folx will start downloading the linked file using all of the aforementioned settings, like file organising. Folx also splits your direct downloads into threads, allowing them to download faster — plus, if a download fails, you can resume it at a later time. It's a really nice way to manage all your downloads (torrents and non-torrents alike) and increase your download speed at the same time.
Folx is a free download for Mac OS X. There is also a Pro version available for $US20, which adds torrent searching, scheduling (the one feature from Transmission that is missing from Folx), smart speed adjustment, and splitting your browser downloads in up to ten threads (instead of just two).