OpenID

From eg

The OpenID .net single login is supported by over 500 services including LiveJournal, Vox and Zooomr and Microsoft (which is integrating its own CardSpace). Like all such efforts it "attempts to create a way to manage large numbers of certificates -- much the way a wallet holds a library card, driver's license, credit cards and grocery loyalty cards." [1]

"When trying to log-in to a service, the user is diverted back to their authentication site, where they enter their password, and the identity site tells the web service that the person is who they say they are." These certificates replace the use of many passwords and supposedly reduces man in the middle attacks.