Transparent Metalinks are when people get the benefit of using Metalinks without knowing they are using them via transparent content negotiation [RFC2295].

People using regular download clients (without Metalink support) will get regular files when they request them. People with Metalink clients can request a regular file, but will receive a .metalink describing that file with all the usual things it would contain (mirror lists, checksums, signatures, etc). These have been mentioned in Peter Poeml's presentation slides and discussed on the list here.

The openSUSE download redirector (aka the MirrorBrain) supports transparent Metalinks.

Currently, aria2 0.13.2, DownThemAll! 1.1 alpha, Retriever 1.1, and Metalink Checker support transparent Metalinks.

Peter Poeml has added some test cases:

I have set up a server with testcases for testing the transparent
negotionation of metalinks in the way we discussed:

 

http://www.poeml.de/transmetalink-test/

 

Assumptions about the client implementation

* The metalink client sends "application/metalink+xml" in the
  Accept request header. Note: "*/*" must also be present. 
  (This is to ensure compliance with RFC 2616, Hayden found at least one
  webserver (IIS) that sends an error message if the "*/*" is omitted.)

* The client checks the returned content type, and if it is
  application/metalink+xml it will handle the metalink
  If it is something else, it handles the response as normal response
  (downloading the file)

* The client does _not_ add the application/metalink+xml to Accept when requesting
  a file _from_ a metalink <url> (thus avoiding loops)

* The client handles external redirects that might send it to the metalink

Basically, click on 4 files and see if what happens is the expected
result :-) 

The README explains the test cases. 

I hope I got them set up correctly; let me know if not, or if you feel
something's missing. 

I hope this helps client authors to verify the implementation!