Metalink is an Open Standard making downloading easier by bundling the various ways (FTP/HTTP/P2P) to get files into one format for easier downloads.
Metalink is a great concept which might be added to the ports infrastructure. Metalink files can be preprocessed in order to retain big mirror lists in bsd.sites.mk. Or an XML guru can convert bsd.sites.mk to an XML version and include it into metalinks in some way. Stub metalinks can be easily created for each port with a simple script.
At this time only www/aria2 has some Metalink support. Aria2 has a few dependencies, but it's completely unnecessary to cram it into the base system. FETCH_DEPENDS would work very nice.
Metalink support can be implemented simultaneously with P2P support, which is long overdue. Aria2 fully supports Metalink 4.0 and other downloaders will soon follow.