OpenID is by far the most comprehensive solution to this issue. All the key players are there: Google, Yahoo, Facebook, et al - and even then it's not as simpl

OpenID is by far the most comprehensive solution to this issue. All the key players are there: Google, Yahoo, Facebook, et al - and even then it's not as simple as it seems. I would put my money on OpenID, nonetheless. It's definitely a mature protocol and gaining traction.