Yadis is a service discovery system allowing relying parties 
(aka identity consumers or membersites) to determine automatically, 
without end-user intervention, the most appropriate protocol to use.

Examples of such services are:

* Single sign-on across web sites
* Profile exchange and form filling
* Blog anti-spam 

Yadis provides the first step for any service that uses identifiers for 
authentication, accountability, privacy controlled data exchange and more. 

WWW: http://search.cpan.org/dist/Net-Yadis/