OpenID

Background

OpenID.net - developer-friendly, home of specifications
OpenID Enabled - home of software modules
Simon Willison's OpenID Info
The Implications of OpenID - Google tech talk by Simon Willison explaining OpenID and some of its implications.
OAuth - An open authentication method that allows users to delegate authorization to a subset of their data.

An OpenID provider authenticates a user's OpenID id, c.f. CAS server.

An OpenID consumer is a web site that uses OpenIDs for authentication, c.f. CAS-enabled application.

It is relatively easy to create an OpenID provider that proxies some other authentication system. If users want to use a particular credential, someone will build the proxy. Organizations ought to consider providing an official service rather than letting an outsider develop a proxy.
OpenID asserts ownership of a URL (OpenID). This is more-or-less equivalent to any authentication system that will send password reminders to an e-mail address (which asserts ownership of that e-mail address).
In some ways, a system that will reset passwords based on a list of questions is the equivalent of single sign on. If the same questions and answers are used on multiple sites, compromise of one may compromise the others.