OpenID: One password to rule them all

OpenID is an open, decentralized, free framework for user-centric digital identity. How it works:

To login to an OpenID-enabled website (even one you've never been to before), just type your OpenID URI. The website will then redirect you to your OpenID Provider to login using whatever credentials it requires. Once authenticated, your OpenID provider will send you back to the website with the necessary credentials to log you in.

OpenID:
http://www.openid.org/

Get an ID at GetOpenID:
https://getopenid.com/