What is the best method for providing means of authentification for multiplayer?

A few thoughts:

 - I don't want to be restricted to a single publishing platform such as Steam.
 - Something like OpenID would be great, but OpenID itself seems to be limited to the world wide web.
 - It shouldn't be anything cryptic, so if you know the email or the user name of your friend you should be able to find him with that.
 - People shouldn't be required to create a additional account just for this one game.
 - [OCS][1] could be used for something like this but afaik it's not supported by any big vendor. So it would be somewhat useless.
 - Things like querying what the players friends are are optional.


 [1]: http://freedesktop.org/wiki/Specifications/open-collaboration-services