D-Link's DNS-323 is already a pretty capable unit with FTP, UPnP AV, iTunes, and print server support, but it's getting even more flexible today with the addition of an official BitTorrent client that'll run independently of your computer. It's certainly not the first NAS we've seen to include everyone's fave copyright, ah, re-appropriation app, but it's certainly one of the cheapest -- you can snag it for under $200 from most retailers.
Source : D-Link updates DNS-323 NAS unit with BitTorrent client - Engadget