iOS (Jailbroken): iTransmission has been the only good BitTorrent option for jailbroken iPhones for a while, but after it was abandoned by the developer it was never updated to run properly on newer operating systems. The project has now been picked up by a new developer and iTransmission has just updated to work on iOS 5 as iTransmission 2.
The last time we looked at iTransmission, it was still in beta and only available through Google Code. The update doesn't add a lot of new features, but it does allow you to download torrents on your iPhone again, which is handy in itself.
It doesn't seem to work with magnet links, but otherwise it can download any torrent you find with the Safari browser. It also has a whole slew of preferences accesible by swiping to the right. The one caveat is that iTransmission 2 requires Winterboard to run, so you'll need to install that before running the app.
iTransmission 2 is a free download. You can find it by searching for iTransmission 2 in the ModMyi repository.