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.
I think everyone is still better off with a password manager system (1password, lastpass, keepass) at this point in time.
OpenID seems to be more troublesome than it is worth and I have avoided registering for one, as well as avoiding those facebook and twitter logins.
It would be interesting to see a largely adopted solution but for some users like me the per site username and password system is still the preferred way to log in.