Speed Download is a new Mac metalink client.

Here's a good explanation of metalink too:

'Metalink makes complex download pages obsolete by replacing long lists of download mirrors and BitTorrent trackers with a single .metalink file. As you might have already guessed, a .metalink file is a file that tells a download manager all the different ways it can download a file. The file itself takes the form of an open XML standard that can list an unlimited number of HTTP and FTP sources as well as BitTorrent trackers and ed2k and magnet links.'
(http://www.downloadsquad.com/2006/08...ftp-downloads/)