The new hot thing in the downloading world. Metalink is a combination of your traditional list of mirrors and a parallel downloader.
If you've ever used a tool like getRight you know the basic idea. Instead of using a single connection for a download, getRight uses a few connections to get different parts of the same file in parallel.
The problem is that you need a list of mirrors that have the same file. Metalink solves this by putting a whole bunch of references to different mirrors in a file, and sending you that list of mirrors. Mirrors can run http, ftp or bittorrent.
Clients can chose the mirrors that are most suitable to them. Combining different protocols is also possible, for example downloading the first half of a file over ftp, and the second half over bittorrent.
The Metalink file also contains a hash of the file, so even if all the original mirrors go down, it's still possible to search for the file on other mirrors / p2p networks.
Another feature is the priorisation of the various links. That way you can instruct the downloaders on which mirrors to try first. For example one could specify that downloaders should try the fast FTP mirror first, if that fails try the bittorrent copy, and only if that fails as well fall back to the primary ftp server.