Windows: Mulve is a portable, single-minded, lightning-fast MP3 search-and-download application. It's not pretty, but it sure does work.
Unlike BitTorrent or other peer-to-peer file-sharing clients (like, say, LimeWire), Mulve isn't peer-to-peer. Instead, it downloads files directly from servers located — well, somewhere. It's all a bit cloak and dagger, which you can read more about over at TorrentFreak. You can't play, preview or manage anything from the app. All you can do is search and download.
Mulve is a free download for Windows only. I wouldn't expect Mulve to be around forever, but in the meantime, it's definitely good at what it's made to do.