CAS vs. OpenID

At the risk of oversimplification, CAS is the preferred choice over OpenID for large, top-down organisations who want to leverage an existing user base. For example, CAS is often deployed by universities that maintain a large, centralized database of users and provide services via a variety of platforms/frameworks.

RubyCAS-Server is also used by many smaller sites. This is likely because the server is considerably easier to deploy than a comparable OpenID solution, especially when one wants to wrap single-sign-on around an existing user base.

A good explanation of the difference between CAS and OpenID is available here: http://stackoverflow.com/questions/2033026/sso-with-cas-or-oauth/3181557#3181557